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A Deutsche Bahn ICE 3 EMU capable of up to 320 km/h (199 mph) in Rhineland-Palatinate, Germany
The Transwa Prospector DEMU, capable of speeds up to 200 km/h (124 mph), in service between Perth and Kalgoorlie in Australia.
Multiple unit trains
Subtypes
Technology
By country
The Bombardier Talent articulated regional railcar
A two-car New South Wales Hunter railcar in Australia

A multiple-unit train (or multiple unit (MU)) is a self-propelled train composed of one or more carriages joined, and where one or more of the carriages have the means of propulsion built in. By contrast, a locomotive-hauled train has all of the carriages unpowered.

An implication of this is that all the powered carriages needs to be controllable by a single engineer or driver, which is a case of the broader concept of multiple-unit train control. In other words, all "multiple units" employ some variation of multiple-unit train control. In the broader context "unit" means any powered rail vehicle, including locomotives (that does not carry cargo) and powered cargo-carrying carriages. In the context of this article, "unit" refers specifically to the latter only (whether the cargo is passengers or some other cargo).

What follows is that if coupled to another multiple unit, all MUs can still be controlled by the single driver,[1] with multiple-unit train control.

Although multiple units consist of several carriages, single self-propelled carriages – also called railcars, rail motor coaches or railbuses – are in fact multiple units when two or more of them are working connected through multiple-unit train control (regardless of whether passengers can walk between the units or not).

History

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South Side Elevated Railroad car #1—one of the cars that Frank Sprague converted to MU operation in Chicago

Multiple-unit train control was first used in electric multiple units in the 1890s.

The Liverpool Overhead Railway opened in 1893 with two-car electric multiple units,[2] controllers in cabs at both ends directly controlling the traction current to motors on both cars.[3]

The multiple-unit traction control system was developed by Frank Sprague and first applied and tested on the South Side Elevated Railroad (now part of the Chicago 'L') in 1897. In 1895, derived from his company's invention and production of direct-current elevator control systems, Frank Sprague invented a multiple-unit controller for electric train operation. This accelerated the construction of electric-traction railways and trolley systems worldwide. Each car of the train has its own traction motors: by means of motor control relays in each car energized by train-line wires from the front car, all the traction motors in the train are controlled in unison.

Design

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Most MUs are powered either by traction motors, receiving their power through a third rail or overhead wire (EMU), or by a diesel engine (DMU) driving a generator producing electricity to drive traction motors.

A MU has the same power and traction components as a locomotive, but instead of the components being concentrated in one car, they are spread throughout the cars that make up the unit. In many cases these cars can only propel themselves when they are part of the unit, so they are semi-permanently coupled. For example, in a DMU one car might carry the prime mover and traction motors, and another the engine for head-end power generation; an EMU might have one car carry the pantograph and transformer, and another car carry the traction motors.

MU cars can be a motor or trailer car, it is not necessary for every one to be motorized. Trailer cars can contain supplementary equipment such as air compressors, batteries, etc.; they may also be fitted with a driving cab.

In most cases, MU trains can only be driven/controlled from dedicated cab cars. However, in some MU trains, every car is equipped with a driving console, and other controls necessary to operate the train, therefore every car can be used as a cab car whether it is motorised or not, if on the end of the train. An example of this arrangement is the NJ Transit Arrows.

Weight reduction

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An advantage of multiple unit trains is that they can be engineered to be lighter than locomotive-hauled trains with separate carriages, using lightweighting techniques to reduce energy use, track wear, and operating costs.[4][5] The term "light-weight train" was first used in the 1930s,[6] with early designs such as the 1934 M-10000 and Pioneer Zephyr in the United States, and Germany's 1932 Flying Hamburger.[7]

Modern light-weight multiple units typically use fewer bogies and distribute traction equipment across the train, improving efficiency and axle loading.[8] However, they require particular design attention for bridge loading and crosswind safety,[9][10] and often include noise and vibration mitigation systems.[11]

Examples from the 21st century include the lightweight Talgo sets used in Spain and exported to Germany and Denmark,[12] India's modern Vande Bharat Express units,[13] and the upcoming TELLi fleet for SNCF regional lines in France.[14]

Passenger multiple units

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Virtually all rapid-transit rolling stock, such as on the New York City Subway, the London Underground, the Paris Metro and other subway systems, are multiple-units, usually EMUs.[citation needed] Most trains in the Netherlands and Japan are MUs, being suitable for use in areas of high population density.[citation needed]

Many high-speed rail trains are also multiple-units, such as the Japanese Shinkansen and the German Intercity-Express ICE 3 high-speed trains. A new high-speed MU, the AGV, was unveiled by France's Alstom on 5 February 2008. It has a claimed service speed of 360 km/h (220 mph). India's ICF announced the country's first high-speed engine-less train named 'train 18', which would run at 250 km/h (160 mph) maximum speed.[15]

Passenger multiple units can be divided into articulated trains and non-articulated trains. The first type can be divided again into the TGV/AGV subtype (which uses Jacobs bogies) and the Talgo subtype.[16]

Freight multiple units

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This section is specifically about multiple units for freight traffic. For freight traffic powered through multiple locomotive "units", see Multiple-unit train control and Distributed power.

Multiple units have been occasionally used for freight traffic, such as carrying containers or for trains used for maintenance. The Japanese M250 series train has four front and end carriages that are EMUs, and has been operating since March 2004. The German CargoSprinter have been used in three countries since 2003.

Comparison to locomotive-hauled trains

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Advantages

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Energy efficiency

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They are more energy-efficient than locomotive-hauled trains. [citation needed]

Gradients

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They have better adhesion, as more of the train's weight is carried on driven wheels, rather than the locomotive having to haul the dead weight of unpowered coaches.

Acceleration

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They have a higher power-to-weight-ratio than a locomotive-hauled train since they don't have a heavy locomotive that does not itself carry passengers, but contributes to the total weight of the train. This is particularly important where train services make frequent stops, since the energy consumed for accelerating the train increases significantly with an increase in weight. Because of the energy efficiency and higher adhesive-weight-to-total-weight ratio values, they generally have higher acceleration ability than locomotive-type trains and are favored in urban trains and metro systems for frequent start/stop routines.

Turnaround times

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Most of them have cabs at both ends, resulting in quicker turnaround times, reduced crewing costs, and enhanced safety. The faster turnaround time and the reduced size (due to higher frequencies) as compared to large locomotive-hauled trains, has made the MU a major part of suburban commuter rail services in many countries. MUs are also used by most rapid transit systems. However, the need to turn a locomotive is no longer a problem for locomotive-hauled trains due to the increasing use of push pull trains.

Failure

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Multiple units may usually be quickly made up or separated into sets of varying lengths. Several multiple units may run as a single train, then be broken at a junction point into shorter trains for different destinations. As there are multiple engines/motors, the failure of one engine does not prevent the train from continuing its journey. A locomotive-drawn train typically has only one power unit, whose failure will disable the train. However, some locomotive-hauled trains may contain more than one power unit and thus be able to continue at reduced speed after the failure of one.

Axle loads

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They have lighter axle loads, allowing operation on lighter tracks, where locomotives may be banned. Another side effect of this is reduced track wear, as traction forces can be provided through many axles, rather than just the four or six of a locomotive. They generally have rigid couplers instead of the flexible ones often used on locomotive-hauled trains. That means brakes/throttle can be more quickly applied without an excessive amount of jerk experienced in passenger coaches. In a locomotive-hauled train, if the number of cars is changed to meet the demand, acceleration and braking performance will also change. This calls for performance calculations to be done taking the heaviest train composition into account. This may sometimes cause some trains in off-peak periods to be overpowered with respect to the required performance. When 2 or more multiple units are coupled, train performance remains almost unchanged. However, in locomotive-hauled train compositions, using more powerful locomotives when a train is longer can solve this problem.

Disadvantages

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Maintenance

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It may be easier to maintain one locomotive than many self-propelled cars. In the past, it was often safer to locate the train's power systems away from passengers. This was particularly the case for steam locomotives, but still has some relevance for casualties than one with a locomotive[clarification needed] (where the heavy locomotive would act as a "crumple zone").

Failure

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If a locomotive fails, it can be easily replaced with minimal shunting movements. There would be no need for passengers to evacuate the train. Failure of a multiple unit will often require a whole new train and time-consuming switching activities; also passengers would be asked to evacuate the failed train and board another one. However, if the train consists of more than one multiple unit they are often designed such that in the event of the failure of one unit others in the train can tow it in neutral if brakes and other safety systems are operational.

Idle trains

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Idle trains do not waste expensive motive power resources. Separate locomotives mean that the costly motive power assets can be moved around as needed and also used for hauling freight trains. A multiple unit arrangement would limit these costly motive power resources to use in passenger transportation.

Passage between units

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It is difficult to have gangway connections between coupled units and still retain an aerodynamic leading front end. Because of this, there is usually no passage between high-speed coupled units, though lower-speed coupled units frequently have connections between coupled units.[citation needed] This may require more crew members, so that ticket inspectors, for example, can be present in all of them. This leads to higher operating costs and lower use of crew resources. In a locomotive-hauled train, one crew can serve the train regardless of the number of cars in the train provided limits of individual workload are not exceeded. Likewise, in such instances, buffet cars and other shared passenger facilities may need to be duplicated in each unit, reducing efficiency.

Flexibility

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Large locomotives can be used instead of small locomotives where more power is needed. Also, different types of passenger cars (such as reclining-seats, compartment cars, couchettes, sleepers, restaurant cars, buffet cars, etc.) can be easily added to or removed from a locomotive-drawn train. This is not so easy for a multiple unit, since individual cars can be attached or detached only in a maintenance facility. This also allows a loco-hauled train to be flexible in terms of number of cars. Cars can be removed or added one by one, but on multiple units two or more units have to be coupled. This is not so flexible.

Noise

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The passenger environment of a multiple unit is often noticeably noisier than that of a locomotive-hauled train, due to the presence of underfloor machinery. The same applies to vibration. This is a particular problem with DMUs.

Obsolescence

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Separating the motive power from the payload-carrying cars means that either can be replaced when obsolete without affecting the other.

By country

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Africa

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Algeria

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A Coradia ZZe trainset from SNTF at Agha Station

Algeria possesses 17 units of the Coradia El Djazaïr, a multiple unit train produced by Alstom. These units are similar to the French version of Régiolis, which belongs to the Coradia family.[17]

South Africa

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Metrorail 10M5 approaching Simon's Town station, Cape Town

Metrorail, which provides commuter rail service in major urban areas of South Africa, operates most services using electric multiple unit train sets of the type 5M2A. These trains are being gradually refurbished and subsequently designated as 10M3 (Cape Town), 10M4 (Gauteng) or 10M5 (Durban). Metrorail services are split into four regions; Gauteng, KwaZulu-Natal, Eastern Cape and Western Cape.

Gautrain, a commuter rail system in Johannesburg, operates with Bombardier Electrostar electric multiple units.

East Asia

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China

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A China Railway High-speed EMU

The concept of multiple unit has entered the horizon of the Chinese since the 6th Speed-up Campaign of China Railway in 2007. With the upgrade of Jinghu Railway, North Jingguang Railway, Jingha Railway and Hukun Railway, and the construction of new Passenger Dedicated Lines (or Passenger Railways) completed, CRH (China Railway High-speed) trains have been put into service, mainly in North and Northeast China, and East China. All these CRH trains are electric multiple units. This was the beginning of the general service of multiple unit trains in China's national railway system.

Far earlier than the introduction of CRH brand, multiple unit trains have been running on all major cities' metro lines in China.

Japan

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A N700 Series Shinkansen set in June 2008

In Japan almost all passenger trains, including the high-speed Shinkansen, are of the multiple-unit (MU) type, with most locomotives now used solely in freight operations. Of the locomotive-hauled passenger services still in operation, the majority are tourist-oriented, such as the numerous steam-hauled trains operated seasonally on scenic lines throughout the country, as well as some of the luxury cruise trains.

Japan is a country of high population density with a large number of railway passengers in relatively small urban areas, and frequent operation of short-distance trains has been required. Therefore, the high acceleration ability and quick turnaround times of MUs have advantages, encouraging their development in this country. Additionally, the mountainous terrain gives the MUs an advantage on grades steeper than those found in most countries, particularly on small private lines many of which run from coastal cities to small towns in the mountains.

Most long-distance trains in Japan were operated by locomotives until the 1950s, but by utilizing and enhancing the technology of short-distance urban MU trains, long-distance express MU-type vehicles were developed and widely introduced starting in the mid-1950s. This work resulted in the original Shinkansen development which optimized all of the EMU's efficiencies to maximize speed. It was introduced upon completion of the Tokaido Shinkansen (literally "new trunk line") in 1964. By the 1970s, locomotive traction was regarded as slow and inefficient, and its use is now mostly limited to freight trains.

From 1999, there have been development efforts in freight EMU technology, but it is currently used only for an express freight service on the Tokaido Main Line between Tokyo and Osaka. The government has been pushing for the adoption of freight EMU technology on energy efficiency grounds in the hope that widespread adoption could assist in meeting CO2 emissions targets. The effort has been principally targeted at express package shipping that would otherwise travel by road.

South Korea

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In South Korea, the KTX-I and KTX-Sancheon, which are still centralized power trains, are the main trains, but the KTX-Eum, which opened in 2021, and the KTX-Cheongryong, which opened in 2024, are the multiple unit.[18][19]

Europe

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Belgium

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The first EMUs have been introduced in Belgium in the 1930s. Several models have followed since then, such as the AM75.

AM75 at the Binche train station (Belgium).

Ireland

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Main article: Multiple units of Ireland

CIÉ introduced its first DMUs, the 2600-class, in 1951.

Russia

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Elektrichka on Yaroslavskiy Rail Terminal, Moscow
Main article: Elektrichka

Elektrichka (Russian: электри́чка, Ukrainian: електри́чка, romanized: elektrychka) is an informal word for elektropoezd (Russian: электропо́езд), a Soviet or post-Soviet regional (mostly suburban) electrical multiple unit passenger train. Elektrichkas are widespread in Russia, Ukraine and some other countries of the former Soviet Union. The first elektrichka ride occurred in August 1929 between Moscow and Mytishchi.

Sweden

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Swedish railroads have been privatized in steps for about 25 years, and today many different companies operate different types of multiple units. A majority of passenger trains today consists of multiple unit trains of which regional traffic exclusively use them.

Switzerland

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The RABe 523 is the most common multiple units on Switzerland, used by almost every S-Bahn.

The Swiss Federal Railways use many multiple units, mainly on regional lines (S-Bahn).

United Kingdom

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Main articles: British electric multiple units and British railcars and diesel multiple units

In the UK, both electric and diesel multiple units are commonplace on suburban and intercity lines, having been introduced from the early 1900s.

Early electric multiple units include the Southern Railway 3Subs, London and North Western 'Oerlikons' and London Underground 1903 Stock. More extensive adoption of multiple units, especially diesel multiple units (DMUs), came about in the 1950s, as British Rail sought to replace steam-hauled services. This would result in several classes of first generation DMUs being built, all similar in design. The vast majority of this design were withdrawn in the 1980s and 1990s, although the Class 121 was used in service with Chiltern Railways from 2003 to 2017.

Southern Class 377/2 377207 at Hemel Hempstead with a train from Milton Keynes Central to East Croydon

Examples of modern multiple units include the Sprinter and Electrostar families, as well as the newer Aventra family.

The London Underground passenger system is operated exclusively by EMUs. Work trains on the Underground employ separate locomotives, some of which are dual battery/live rail powered.

In Northern Ireland the majority of passenger services have been operated by diesel multiple units since the mid-1950s under the tenure of both the Ulster Transport Authority (1948–1966) and Northern Ireland Railways (since 1967).

Oceania

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Australia

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The first multiple unis in Australia were the Tait trains, wooden bodied Electric Multiple Unit train that operated in Melbourne, Victoria. They were originally introduced as steam locomotive hauled carriages but were converted to electric traction from 1919 during Melbourne's electrification project.[21]

South Asia

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India

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Main article: Multiple units of India

Indian Railways has recently introduced a semi-high-speed EMU named Vande Bharat Express, capable of running at 183 km/h (114 mph). And it continues to use diesel and electrical multiple units on its national network. All suburban and rapid transit lines are served by EMUs.

Southeast Asia

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Indonesia

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Indonesia uses diesel since 1976 and electric MUs since 1925. Most of these MUs were built in Japan.

Philippines

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The Manila Railroad Company (MRR) acquired its first multiple units in the 1930s. The locally-built MC class was initially powered by gasoline and was changed to diesel during World War II. Both the MRR and its successor, the Philippine National Railways (PNR), has since acquired various classes of diesel multiple units. All multiple units owned by MRR and all of the older MUs of the PNR were built by Japanese firms. On the other hand, its newer rolling stock were built in South Korea and Indonesia. There will also be DMUs that will be built in China.[22]

The first electric multiple units were acquired in 1984 for the LRT Line 1 built by La Brugeoise et Nivelles in Belgium.[23] The first EMUs to be used outside of rapid transit will enter service between 2021 and 2022.[24]

North America

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New Jersey transit Stadler GTW DMU used on the River Line
See also: Commuter rail in North America

Most trains in North America are locomotive-hauled and use Multiple Unit (MU) control to control multiple locomotives. The control system of the leading locomotive connects to the other locomotives so that the engineer's control is repeated on all the additional locomotives. The locomotives are connected by multi-core cables. The Railway Technical Website, vol. US Locomotive MU Control. This does not make these locomotives MUs [dubious – discuss]for the purposes of this article. See locomotive consist.

However, commuters, rapid transit, and light rail operations make extensive use of MUs. Most[citation needed] electrically powered trains are MUs.

The Southeastern Pennsylvania Transportation Authority (SEPTA) Regional Rail Division uses EMUs almost exclusively — the exception being some of its peak express service. New Jersey Transit service on the Northeast Corridor Line is split between electric locomotives and EMUs.

M2, M4, M6 and M8 EMUs which operate on the New Haven Line of Metro-North Railroad, are “multi-system” meaning they can draw power from either the third rail or from overhead lines. This allows operation under the wires between Pelham, NY and New Haven, CT, a section of track owned by Metro North but shared with Amtrak's Northeast Corridor service, and on third rail between Pelham and Grand Central Terminal. EMUs are used on AMT's Montreal/Deux-Montagnes line.

DMUs are less common, partly because new light rail operations are almost entirely electric, with many commuter routes already electrified, and also because of the difficulties posed by Federal Railway Administration rules limiting their use on shared passenger/freight corridors. When the Budd RDC was developed following World War II, it was adopted for many secondary passenger routes in the United States (especially on the Boston and Maine Railroad) and Canada. These operations generally survived longer in Canada, but several were abandoned in the Via Rail cutbacks of the early 1990s. One that survives is Victoria - Courtenay train on Vancouver Island. DMU use in Canada has been resurrected in recent years, beginning with the opening of Union Pearson Express in 2015.

While most DMUs need to comply with strict FRA crash requirements for simultaneous operation with freight railways, European-style DMUs are used with timesharing arrangements on several rail lines, including the RiverLINE in New Jersey. Only a handful of manufacturers in the United States produce or have produced FRA-compliant DMUs, including Colorado Railcar (now US Railcar) and Nippon Sharyo/Sumitomo Corporation. NJ Transit has experimented with this DMU on the Princeton Branch line. In August 2006 it was announced that Amtrak wants the State of Vermont to experiment with DMUs on the state-subsidized Vermonter line from New Haven north to St. Albans to replace the less efficient diesel locomotive trainsets currently used.

MU streetcars were used in Toronto by the Toronto Transportation Commission (later Toronto Transit Commission) from 1949 to 1966 using 100 PCC A-7 built by St. Louis Car Company and Canadian Car and Foundry.[25] These two car units ran along the Bloor Street route only beginning in 1950 and ceased operations after the opening of the Bloor–Danforth subway line in 1966. The A-7 units were later converted to single use.

See also

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References

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Notes

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Multiple unit

View on Grokipedia
from Grokipedia
A multiple unit (MU), also known as a multiple-unit train, is a self-propelled composed of two or more carriages joined together, in which propulsion equipment is installed on most or all vehicles, allowing motive power to be distributed rather than concentrated in a single . This design enables centralized control from a single cab via multiple-unit train control systems, allowing efficient operation of passenger or freight . The multiple-unit system originated in the late as part of the advancement of electric traction in urban railways. In 1897, inventor Frank J. Sprague successfully implemented the first multiple-unit control system on the South Side Elevated Railroad in , , enabling multiple electric cars to be operated as a single unit from the leading cab. This innovation, building on Sprague's earlier work with electric elevators in 1891, revolutionized by facilitating longer trains with distributed power, and it was commercialized through the Sprague Electric Company before being acquired by in 1902. Multiple units are categorized primarily by power source, including electric multiple units (EMUs) that draw power from overhead wires or third rails for urban and suburban services, and diesel multiple units (DMUs) that use onboard diesel engines for non-electrified routes. EMUs and DMUs are prevalent in , regional passenger services, and applications worldwide, offering benefits such as faster acceleration due to distributed propulsion, lower operating costs compared to locomotive-hauled trains, and improved energy efficiency. These advantages make multiple units particularly suitable for high-frequency, medium-capacity operations in dense urban environments.

Introduction

Definition and principles

A multiple unit (MU) is a self-propelled railway trainset in which propulsion equipment is distributed across most or all cars, typically composed of one or more powered cars, which may include unpowered trailer cars semi-permanently coupled to function as a single integrated vehicle, with motive power distributed across multiple axles or bogies rather than concentrated in a separate locomotive. This design enables the entire trainset to operate without an external hauling locomotive, distinguishing MUs from traditional locomotive-hauled consists. The fundamental operating principles of multiple units revolve around distributed traction and centralized control to enhance performance. Power distribution across several powered cars improves adhesion and traction by spreading weight and motive force evenly, leading to better acceleration and up to 5-10% greater energy efficiency compared to centralized power systems. Control is centralized through electrical jumper cables connecting the cars, allowing a single operator in any driving cab to command all propulsion, braking, and auxiliary systems simultaneously via multiple-unit train control (MU control) technology. In modern implementations, wireless systems may supplement or replace cables for enhanced flexibility. Key components of a multiple unit include power cars, which house the motors, engines, or pantographs and often feature driving cabs at one or both ends for operator control; trailer cars, which provide additional passenger or freight capacity without onboard motive power; and specialized semi-permanent couplers that mechanically and electrically link cars for seamless operation. Driving cabs are typically located at the ends of the outermost power cars, equipped with controls for the entire trainset. The term "multiple unit" originated in the late from early electric railway terminology, specifically Frank J. Sprague's invention of multiple-unit train control, which adapted elevator control principles to enable synchronized operation of multiple self-propelled cars from a single point. This innovation emphasized the "multiple" motors and unified control across units, marking a shift toward integrated train designs in the early .

Classification systems

Multiple units are primarily classified by their power source, which determines their mechanism and operational suitability. Electric multiple units (EMUs) consist of self-propelled carriages that derive motive power from external electrical sources, such as overhead wires or third rails, enabling efficient operation on electrified networks. Diesel multiple units (DMUs) rely on on-board diesel engines directly driving the wheels through mechanical or hydraulic transmission, offering flexibility for non-electrified routes. Diesel-electric multiple units (DEMUs) integrate diesel engines with generators to produce for traction motors, combining the autonomy of diesel power with the efficiency of electric drive systems. Hybrid and battery-powered units represent further evolution, incorporating rechargeable batteries alongside diesel or electric sources to optimize energy use and reduce emissions, as seen in configurations that switch between power modes based on route conditions. Functionally, multiple units are categorized according to their service role and performance characteristics. Short-haul commuter units are designed for high-frequency urban and suburban operations, emphasizing rapid acceleration and capacity for daily passengers. Long-distance units prioritize comfort and speed for regional connectivity, often featuring amenities for extended journeys. High-speed variants operate at commercial speeds of at least 250 km/h on dedicated infrastructure, as defined by the (UIC), to facilitate efficient long-haul travel. and multiple units serve urban transit needs with lower speeds and street-running capabilities, focusing on integration with city environments. Regional standards govern multiple unit design and , reflecting variations in and regulatory frameworks. In , the UIC establishes norms for axle arrangements (e.g., using notations like 1A-A1 for powered wheelsets) and ensures compatibility on the standard 1,435 mm gauge, promoting cross-border operations. In , the Association of American Railroads (AAR) applies similar axle classification principles alongside specifications for self-propelled passenger cars, also on 1,435 mm gauge, to maintain safety and interchangeability across networks. These standards facilitate gauge compatibility on shared global tracks while addressing local requirements for loading, braking, and . As of 2025, emerging classifications include hydrogen fuel cell multiple units, which use fuel cells to generate for propulsion, exemplified by the Zero-Emission Multiple Unit (ZEMU) trainset deployed in for clean, non-electrified routes. Autonomous multiple units, incorporating advanced automation for driverless operation, are advancing toward commercialization, with prototypes targeting regional passenger services by the mid-2020s.

Historical development

Early innovations (1900–1945)

The early 20th century marked the beginning of multiple unit (MU) development, with pioneering self-propelled railcars emerging as alternatives to steam-hauled trains, particularly for branch lines and urban services. In the , the North Eastern Railway introduced the world's first petrol-electric railcars in 1903, known as autocars, featuring a 100 hp petrol engine driving a British Westinghouse generator and 64 hp DC traction motors; these single-car units seated 50 passengers and operated until 1931 on routes to Scarborough and . In the United States, early innovations included gas-powered railcars like the McKeen motor cars introduced by the Union Pacific in 1905, which used 4-cylinder engines to power lightweight, single-car units for short-haul services, achieving speeds up to 60 mph and influencing later diesel designs. These developments addressed the need for economical operation on low-traffic lines, where full locomotives were impractical. Key milestones in the 1930s advanced MU technology through and diesel , enabling faster and more efficient services. In , projects led to the deployment of the first high-speed electric multiple units, while pioneered diesel multiple units with the 1933 , dubbed the Flying Hamburger, a two-car diesel-electric trainset that reached 160 km/h on the Berlin-Hamburg route, covering 290 km in under 3 hours and setting records for non-stop diesel services. Wartime adaptations during further emphasized efficiency, as resource shortages prompted railways in and to prioritize MUs for their lower fuel consumption and smaller crews; for instance, British Railways repurposed existing diesel railcars for essential freight and passenger duties, reducing reliance on scarce . Early multiple units encountered significant challenges related to power distribution and reliability, limiting their widespread use. Power transmission via generators and traction motors often proved unreliable under varying loads, as seen in the 1903 North Eastern Railway autocars, where initial 85 hp engines were upgraded to 100 hp units in 1904 due to frequent breakdowns. In the United States, the 1910 conversions of New York subway cars to improved multiple-unit control systems addressed issues with inconsistent power distribution in longer consists, enhancing synchronization across cars but highlighting the era's limitations in electrical integration for urban rapid transit. These hurdles were compounded by mechanical wear on early internal combustion engines, necessitating frequent maintenance and restricting operations to lighter duties. The global spread of multiple units during this period was concentrated in urban centers of and , where they offered substantial cost savings over by integrating propulsion into passenger cars, thus eliminating separate motive power and reducing labor needs by up to 50% on short routes. Adoption began with electric MUs in subways like New York's Interborough Rapid Transit system, which opened in 1904 with 500 steel-frame cars featuring underfloor motors and third-rail collection, setting a model for efficient mass transit. By the 1940s, diesel and electric variants had proliferated in cities such as and , prioritizing reliability improvements for commuter services amid growing networks.

Post-war expansion (1946–1990)

Following , the reconstruction of war-damaged rail networks in and drove the rapid adoption of multiple units to restore efficient passenger services on branch lines and urban routes. In the , introduced its first generation of diesel multiple units (DMUs) in the mid-1950s as a cost-effective alternative to , with the Metro-Cammell-built Class 101 entering service in 1957 to serve regional and suburban lines. Similarly, in , the (JNR) accelerated electrification and deployed electric multiple units (EMUs) for intercity and urban commuting, starting with lightweight high-performance models in 1951 that reduced vehicle weight by about 25% using high-tensile steel, followed by the Kodama limited express EMU on the Tokaido Main Line in 1958. These developments addressed postwar shortages of fuel and while supporting economic recovery and population growth in urban areas. Technological advancements in the and further standardized multiple unit operations, enabling easier formation of longer trains. In , efforts in the and aimed to standardize coupling systems compatible across networks, facilitating multiple unit control through jumper cables for electrical and pneumatic connections, though full adoption varied by country. In , the Société Nationale des Chemins de fer Français () pioneered high-speed prototypes, including the gas turbine-powered TGS in 1967, which reached 252 km/h in 1971, and the TGV 001 experimental multiple unit in 1972, achieving 318 km/h during testing; the prompted a shift to electric propulsion, leveraging nuclear-generated power for future high-speed EMUs. Economic pressures, such as declining freight volumes and the need to maintain uneconomic branch lines, spurred the transition to diesel and electric multiple units globally during the . In , the introduced the locally built Redhen diesel railcars in 1955, with production continuing into the , to replace aging steam-hauled suburban services around amid rising operational costs and competition from . By the 1980s, adoption peaked worldwide as networks expanded to meet surging urban demand; in , deployed advanced chopper-controlled EMUs in starting with trials in 1981 under a collaboration with the , transitioning to 12-car formations by 1988 to handle growing commuter traffic on the 1.5 kV DC suburban lines. In the , the Riga Car Building Factory mass-produced the ER2 series electric multiple units from the early onward, with ongoing builds through the 1980s supporting extensive suburban and handling peak passenger loads on DC-electrified routes.

Modern advancements (1991–present)

Since the 1990s, the integration of digital signaling systems has transformed multiple unit operations, enabling higher automation and safety levels. The (ETCS), standardized under the Agency for Railways, saw its first specifications released in 1996, with initial onboard installations in multiple units occurring in the early across . By the , ETCS Level 2 was widely adopted for mainline multiple units, such as in the UK's East Coast Digital Programme, which equipped over 700 locomotives and multiple units for real-time route information and automated protection by 2024. Complementing this, (CBTC) emerged in urban settings during the , utilizing radio communications for continuous positioning and moving-block operations in multiple units. A seminal implementation was New York City's Canarsie Line in 2006, where CBTC reduced headways and increased capacity on subway multiple units by enabling automated supervision. Sustainability initiatives gained prominence in multiple unit design from the 2000s, driven by regulatory pressures for reduced emissions. Regenerative braking systems, which recapture during deceleration and feed it back to the power supply, became standard in electric multiple units, achieving energy savings of 10% to 45% in metro applications depending on route profiles. This technology, integrated into traction inverters, minimizes waste heat and supports grid stability, with widespread adoption in European and Asian fleets by the 2010s. Low-emission hybrids further advanced this focus, exemplified by Alstom's Coradia iLint, the world's first hydrogen-powered passenger multiple unit, which entered commercial service in Lower Saxony, , in September 2018. The iLint uses fuel cells to generate electricity from , emitting only and enabling zero-CO2 operation on non-electrified lines up to 1,000 km, with over 200,000 km accumulated in service by 2022. High-speed multiple units evolved significantly in the 2000s, prioritizing capacity and modularity for extended consists. China's CRH (China Railway High-speed) series, launched in 2007 on the Beijing-Tianjin intercity line, marked the rapid expansion of the world's largest high-speed network, incorporating modular designs derived from international technologies like Japan's for efficient assembly and maintenance. By the 2010s, CRH trains operated at speeds up to 350 km/h, with standardized platforms allowing flexible coupling of up to 16 cars to meet peak demand. In Japan, the N700S entered service in July 2020 on the Tokaido and lines, enhancing modularity through interchangeable components and reduced air resistance for improved energy efficiency at 300 km/h. Its silicon carbide-based traction system supports longer consists of up to 16 cars while maintaining earthquake-resistant features. Into the 2020s, battery-electric multiple units addressed electrification gaps on legacy lines, alongside AI-driven maintenance. In the UK, conversions like Vivarail's Class 230, adapted from stock, demonstrated viability for non-electrified routes, achieving a 320 km record in 2025 using battery power supplemented by . These units enable zero-emission operation at speeds up to 100 km/h, with batteries charged via overhead lines or pantographs where available. AI predictive maintenance has concurrently reduced downtime, with approximately 60% of global railway companies having implemented at least one AI at scale as of 2023 to analyze sensor data for fault prediction, including in multiple units.

Technical design

Propulsion and power systems

Electric propulsion systems in multiple units primarily rely on external power sources delivered through overhead catenary or third-rail infrastructure. The overhead catenary system uses a network of wires suspended above the tracks, typically at 25 kV AC in European mainline and high-speed applications, allowing efficient transmission over long distances with reduced energy losses compared to lower-voltage systems. Pantographs, mounted on the roof of the train, maintain continuous contact with the catenary via a spring-loaded, diamond-shaped frame that adjusts to track irregularities and speeds up to 300 km/h, ensuring stable power collection while minimizing wear. In contrast, third-rail systems employ a rigid conductor rail alongside the track, energized at around 750 V DC, which is common in urban and metro networks for its lower installation costs and reduced vulnerability to weather, though it limits speeds due to mechanical contact stresses. Diesel propulsion in multiple units incorporates onboard engines, often medium-power units rated at 250–560 kW derived from or industrial designs, coupled with torque converters for smooth from standstill. These systems, including diesel-hydraulic variants with hydrodynamic torque converters and multi-speed transmissions, achieve peak efficiencies up to 95% in hydro-mechanical configurations, though overall fuel consumption averages 2.0–2.5 kg/km for typical regional services. Hybrid diesel-battery setups integrate lithium-ion or storage to capture braking energy, enabling engine shutdown during low-demand phases like station stops, which reduces fuel use by 6–13% and CO₂ emissions proportionally while operating the in its optimal 40% efficiency range. Battery-electric multiple units (BEMUs) use onboard rechargeable batteries, typically lithium-ion packs with capacities of 1–4 MWh, to power traction motors for non-electrified routes, offering zero-emission operation over distances up to 100–200 km per charge as of 2025. These systems support fast charging at stations and to extend range, with examples including Stadler's FLIRT Akku trains in . Hydrogen fuel cell multiple units (HMUs) employ fuel cells generating electricity from stored in compressed tanks, achieving ranges over 600 km and refueling times under 15 minutes; the ZEMU, the first in the US, debuted in in September 2025, demonstrating hybrid fuel cell-battery integration for regional services. Power distribution in multiple units employs multiple , typically one per , to provide even traction across the consist, enhancing stability and . This distributed arrangement generates F=m×aF = m \times a, where FF is the total pulling force, mm is the , and aa is , with multiple minimizing slip on gradients by equalizing demands and improving overall grip. Energy efficiency in these systems is quantified by the basic consumption formula E=P×tηE = \frac{P \times t}{\eta}, where EE is energy used, PP is power output, tt is time, and η\eta is the system efficiency factor, often ranging from 0.76 for electric traction to 0.37–0.40 for diesel variants. Distributed power elevates η\eta by optimizing load sharing, reducing losses from uneven adhesion, and enabling regenerative braking recovery of up to 20–30% of input energy in hybrid configurations. To derive this, start with total energy input as P×tP \times t, then divide by η\eta (product of motor, transmission, and auxiliary efficiencies) to yield net consumption, where improvements from distribution stem from lower peak loads per motor, as validated in traction models for commuter rail.

Control and coupling mechanisms

Control systems in multiple units enable coordinated operation of , braking, and auxiliary functions across coupled vehicles, typically through master-slave configurations where the lead vehicle dictates commands to trailing ones. In traditional setups, particularly for diesel-electric locomotives and early electric multiple units in , this is achieved via multi-pin jumper cables that transmit electrical signals for , direction, , and sanders. The standard (AAR) system uses 27-pin jumper cables with cast aluminum plugs, where specific pins handle functions like power reduction (pin 1, 0-74 V DC) and generator field excitation (pin 6), ensuring synchronized operation when trailing units are set to "trail" mode via pneumatic hoses for brake control. Variations exist, such as 13-pin or 15-pin configurations in some older or regional systems for simplified control in electric multiple units, while electro-pneumatic "train wires" in Australian railmotor units activate magnet valves for and transmission across up to five cars, with interlocks preventing gear shifts unless at idle. Modern advancements since the have shifted toward wireless train control and monitoring systems (TCMS), reducing reliance on physical jumpers for enhanced flexibility and reduced maintenance. Radio-based TCMS, part of the (TCN) evolution under initiatives like Europe's Roll2Rail project, enables bidirectional wireless communication for safety-critical functions, allowing seamless data exchange between vehicles without wired connections and supporting concepts for dynamic formations. These systems use protocols compliant with IEC 61375 standards, integrating with for real-time monitoring, though hybrid wired-wireless setups persist for fail-safe redundancy in high-speed multiple units. Coupling mechanisms facilitate both mechanical linkage and functional integration, with types varying by region to support multiple working. In , automatic couplers like the Janney (AAR Type E) or Tightlock variants are prevalent for multiple units, enabling semi-automatic shunting while incorporating electrical and pneumatic interlocks to connect brake hoses (e.g., MR and BC lines) and jumper cables, ensuring air pressure equalization and electrical continuity for unified control. European systems often employ drawbar arrangements or compact automatic couplers such as Scharfenberg (Scharfenbergprofil) on non-freight multiple units, which integrate mechanical drawbars with multi-function plugs for electrical (up to 13 contacts) and pneumatic (brake and auxiliary air) connections, allowing one-person coupling with automatic alignment for seamless operation across borders under UIC standards. These interlocks prevent uncoupling under load and verify connections before movement, enhancing safety during formation. [Note: Wikipedia not cited, but concept from PDF] Safety features are integral to control and , prioritizing operation to mitigate human error or system faults. The deadman's handle, a vigilance device requiring periodic operator acknowledgment, triggers emergency braking if released or unacknowledged, commonly integrated into multiple unit cabs since the early to monitor driver alertness across formations. Automatic Train Protection (ATP) systems enforce speed limits and movement authorities via onboard transponders or radio, automatically applying brakes if overspeed or signal violations occur, as seen in U.S. (FRA) guidelines for high-speed guided transport where ATP uses two-out-of-three voting logic for . braking logic ensures brakes engage on power loss or fault detection, with electro-pneumatic valves defaulting to applied positions and dynamic brakes supplementing friction systems for uniform deceleration, preventing propagation of failures in coupled units. Scalability allows multiple units to form longer consists, typically up to 12 cars, through standardized protocols that distribute control signals for uniform and braking. In commuter services like those on Japan's E259 series or U.S. electrics, units couple via compatible interlocks to extend to 12-car lengths, with TCMS or jumper systems propagating commands to maintain consistent power output and avoid slack action, limited by traction capacity and signaling to ensure stability at speeds up to 200 km/h. These protocols, often governed by national standards like those from the (UIC), enable reversible formations where any unit can lead, supporting operational flexibility without compromising safety.

Structural and aerodynamic features

Multiple units (MUs) employ to optimize structural integrity while minimizing weight, primarily using lightweight aluminum alloys and composite materials for car bodies. Aluminum extrusions and panels form the primary structure, often welded or riveted into a design that distributes loads across the shell rather than relying on a separate frame, enabling significant weight savings. For instance, the regional MU features a welded integral aluminum construction that reduces overall vehicle mass compared to traditional designs. Composites, such as carbon fiber reinforced polymers, are increasingly integrated in hybrid structures for further reductions; a study on (EMU) car bodies using aluminum-carbon fiber hybrids achieved up to 20% weight savings through optimized material distribution. These techniques result in MUs being approximately 20-30% lighter than equivalent locomotive-hauled consists, improving energy efficiency and without sacrificing strength. Aerodynamic features in MUs focus on minimizing drag to enhance high-speed performance and reduce , with designs emphasizing streamlined shapes and smooth gangway connections between cars. The typically incorporates rounded, tapered profiles to deflect airflow effectively, while inter-car gaps are sealed with or fairings to prevent . testing is standard for validating these elements, ensuring compliance with performance criteria for speeds exceeding 200 km/h. High-speed MUs like the series achieve drag coefficients (Cd) below 0.3 through such optimizations, with values around 0.25 for streamlined configurations, leading to drag reductions of 10-15% compared to less refined shapes. These features are particularly critical for systems, where uniform across the train length maintains stability. Integration of passenger amenities in MU structures prioritizes modularity to support accessibility without weakening the overall frame, allowing interiors to be reconfigured for diverse needs. Modular panels and flooring systems enable easy installation of features like wider aisles, priority seating, and wheelchair spaces, while maintaining structural rigidity through reinforced mounting points. In the United States, MUs must comply with Americans with Disabilities Act (ADA) standards, including level boarding, accessible restrooms, and securement areas for mobility aids, achieved via standardized modular kits that fit within the car's load-bearing skeleton. Examples include Alstom's Coradia series, where modular interiors provide flexible layouts for ADA-compliant spaces, such as dedicated low-floor sections, ensuring seamless passenger flow. This approach balances comfort enhancements with the integrity of the monocoque or semi-monocoque body. Durability in MUs incorporates standards and control to protect occupants and ensure longevity under operational stresses. In , EN 15227 mandates energy absorption in collision scenarios, requiring deformable front-end structures with crush zones that limit deceleration to survivable levels, applicable to both locomotives and MU driving cars. These standards classify vehicles into design categories based on service type, with high-speed MUs featuring reinforced end underframes to absorb impacts up to 36 km/h. For , especially in configurations where motors are spread across cars, multiple dynamic vibration absorbers (DVAs) are installed on floors and bogies to mitigate resonant frequencies, reducing passenger discomfort by up to 50% at speeds over 250 km/h. Such measures enhance ride quality while preserving the lightweight composite or aluminum framework.

Operational types

Passenger multiple units

Passenger multiple units (PMUs) are self-propelled rail vehicles designed primarily for transporting passengers, emphasizing efficiency, comfort, and rapid acceleration for frequent stops in urban and regional services. These units, often electric multiple units (EMUs), integrate propulsion, passenger accommodation, and control systems within the same consist, allowing for flexible formations without separate locomotives. Configurations vary by service type, with commuter PMUs prioritizing high passenger throughput and intercity variants focusing on longer-distance travel amenities. Commuter PMUs typically feature high-capacity standing areas to accommodate peak-hour crowds, with 4 to 6 doors per car to facilitate quick boarding and alighting. For instance, modern designs like Alstom's Multilevel cars for commuter services include two sets of extra-wide doors per side, enabling efficient passenger flow in dense urban environments. In contrast, intercity PMUs incorporate reclining seats arranged in 2+2 or 2+1 layouts and onboard catering facilities, such as vending areas or sections, to enhance journey comfort over extended routes. Comfort features in passenger multiple units have evolved to meet modern expectations, including advanced (HVAC) systems that maintain optimal cabin temperatures and air quality even during high occupancy. integration became a standard amenity in many PMUs during the , providing passengers with reliable for work or , as seen in high-speed designs like China's CRH series. Accessibility enhancements, such as automatic ramps and low-floor entry points, ensure compliance with disability regulations and inclusive travel; Japan's E5 series , for example, offers spacious interiors with priority seating and elements for enhanced passenger experience. Capacity metrics for multiple units typically range from 1,500 to 2,000 per 8-car set, combining seated and standing accommodations to maximize throughput while adhering to loading gauges. The multiple doors and in PMUs contribute to dwell time reductions compared to locomotive-hauled trains, improving schedule adherence in busy corridors. Double-deck configurations, like those in ’s double-deck EMUs, boost capacity with up to 1,000 per 8-car set including standing, while single-level units predominate on regional lines for simpler infrastructure compatibility and lower construction costs.

Freight multiple units

Freight multiple units (FMUs) are self-propelled rail formations designed specifically for goods transport, featuring permanently coupled cars with integrated propulsion systems to handle heavy cargo loads efficiently. Unlike traditional locomotive-hauled trains, FMUs distribute motive power across the consist, enabling better traction and control for demanding freight operations. These units address the need for reliable, high-capacity hauling in sectors like intermodal container transport and bulk goods, with adaptations focused on durability and logistics optimization. True FMUs remain niche, with historical examples including German DB Type 798 diesel railcars from the 1990s for lightweight freight. Design adaptations in FMUs emphasize structural reinforcements to support intense cargo demands, such as reinforced underframes capable of withstanding the stresses of loading and unloading. These underframes incorporate longitudinal and transverse members to distribute weight evenly, preventing deformation under heavy payloads like stacked intermodal . For heavy-haul applications, configurations using multiple locomotives placed along the train improve starting traction and reduce slack action in long consists; in , operators have used setups for and trains, enhancing performance on steep gradients. Operational advantages of such configurations include streamlined terminal handling, as fixed formations or multi-loco setups eliminate the need for locomotive repositioning during shunting, reducing turnaround times and labor requirements. This allows for quicker assembly and dispatch of freight trains, minimizing delays in busy yards. In , since the , Vectron multi-system have been deployed in multiple-unit configurations for cross-border freight, enabling efficient operations without decoupling, as seen in DB Cargo's international services. FMUs or trains typically consist of 4 to 10 coupled units, optimized for specific cargo types such as s for double-stacked containers or flatbeds for oversized loads like machinery and . formations, for instance, feature depressed centers to lower the center of , accommodating 40- or 45-foot containers while maintaining stability at high speeds. Emerging electric FMUs further support decarbonization efforts; Wabtec's FLXdrive battery-electric locomotives, introduced in hybrid configurations in 2023, integrate into freight consists to cut diesel use by up to 11% and reduce , as demonstrated in trials with Australian mining operations. Key challenges in FMU operations involve managing higher loads, often reaching 25 tonnes per axle to maximize capacity, which accelerates track wear and requires enhanced . Mitigation strategies include advanced rail profiles and systems to minimize and corrugation, ensuring long-term integrity in heavy-haul corridors.

Specialized and hybrid units

Hybrid multiple units, also known as bi-mode units, integrate both electric and diesel systems to operate seamlessly on electrified and non-electrified track sections, enhancing flexibility in mixed infrastructure networks. The Class 800, introduced in 2017 by for operators like Great Western Railway and , exemplifies this design; it employs electric traction under overhead wires and onboard diesel generators for unelectrified routes, achieving speeds up to 125 mph in electric mode and supporting the UK's . Specialized multiple units serve niche applications beyond standard passenger or freight services, including and . Self-propelled rail grinders, such as those from RailTechnology's ST series, function as compact multiple-unit configurations equipped with up to 24 grinding stones to restore rail profiles and remove irregularities, operating at speeds of 2-16 km/h for grinding while self-propelling at up to 70 km/h between sites. Track inspection multiple units, like modified diesel multiple units (DMUs) used by , incorporate advanced sensors for geometry and defect monitoring; for instance, the Class 153 inspection train enables real-time visual and ultrasonic assessments without dedicated locomotives. shuttle variants, such as the refurbished Class 387 electric multiple units on , provide high-frequency, non-stop service between London Paddington and , featuring dedicated interiors for 374 passengers per 160-meter train and business-class seating. In , Switzerland's panoramic trains operated by the on routes like the offer large-windowed coaches for alpine views through UNESCO-listed landscapes. Military adaptations of multiple units have emerged in conflict zones to protect logistics and personnel. During the 2022 Russian invasion of , both sides modified existing rail vehicles into armored trains; adapted civilian locomotives and cars with plating for secure supply transport, while Russian forces deployed units like the , a self-propelled armored train with anti-aircraft defenses for reconnaissance and repair under fire. In research, multiple-unit testbeds advance technology; Japan's 's Yamanashi test line uses superconducting () multiple units to validate speeds exceeding 600 km/h, with extensive sensor integration for stability and aerodynamics testing. As of 2025, future concepts explore autonomous hybrid multiple units combining freight and passenger functions to optimize shared high-speed corridors. In , research optimizes integrated freight-passenger electric multiple units on existing lines, allowing dynamic to reduce congestion and costs, with trials focusing on unmanned operations for efficiency. Additionally, China's first fully autonomous standard multiple units, debuted at MetroTrans 2025, incorporate AI for driverless , paving the way for hybrid applications in mixed urban-rural networks.

Comparative analysis

Advantages

Multiple units offer significant energy efficiency advantages over traditional locomotive-hauled trains due to their systems, which minimize transmission losses and enable lighter overall designs. For instance, electric multiple units (EMUs) can achieve significant energy savings compared to equivalent diesel-electric locomotive-hauled configurations, primarily through optimized power distribution and reduced weight per seat. Diesel multiple units (DMUs) similarly demonstrate improved efficiency, with examples showing 5-10% savings in single-vehicle operations and 1-2% fleet-wide benefits from better space utilization and lower mass per passenger. further enhances this, allowing recovery of 10-45% of braking energy in metro and commuter applications, which directly reduces net energy demand. Performance metrics of multiple units surpass those of locomotive-hauled trains, particularly in and handling, thanks to traction motors distributed across multiple axles. This enables rates often around 0.8-1.2 m/s², compared to typical 0.3-0.5 m/s² for locomotive-hauled passenger consists where power is concentrated at the front. The multi-axle traction improves and pulling power on inclines, reducing travel times on routes with stops or elevation changes without requiring oversized locomotives. Operationally, multiple units provide gains through simplified procedures and reliability enhancements, as there is no need for locomotive-trailer coupling, leading to shorter turnaround times—often 5-10 minutes versus 30 minutes for loco-hauled setups requiring uncoupling and repositioning. This bidirectional capability and reduced interfaces also lower failure rates by eliminating mechanical junctions prone to wear. Additional benefits include even loads, typically 18-22 tonnes, which distribute weight uniformly and minimize track maintenance needs compared to the uneven loading from concentrated mass in hauled trains. Integrated designs further contribute to reduced levels, with EMUs recording interior levels as low as 73 dB during operation, benefiting passenger comfort and urban deployment.

Disadvantages

Multiple units suffer from elevated maintenance requirements owing to the distributed nature of their propulsion and control systems, which introduce greater complexity across each car compared to centralized locomotive-hauled configurations. The distributed propulsion may require more maintenance efforts on electrical and mechanical components in every car, rather than concentrating these in a single locomotive. Multiple units often have higher initial acquisition costs due to the need for propulsion systems in multiple cars. Reliability challenges arise because a failure in any single car—such as an electrical or fault—can compromise the entire trainset, necessitating the removal of the whole unit from service for repairs and potentially causing significant operational disruptions. This contrasts with locomotive-hauled trains, where issues are often isolated to the power car, allowing the rest of the consist to remain operational. Additionally, in , some multiple unit designs feature limited or restricted passages between cars to maintain structural integrity and safety, which can complicate rapid evacuation and require reliance on end doors or emergency exits. Flexibility is another limitation, as multiple units are generally built as fixed-length sets that are difficult to reconfigure by adding or removing cars to match fluctuating demand, leading to underutilized capacity on low-demand routes where powered cars run idle without contributing to revenue. This rigidity can result in inefficient , particularly for operators serving variable traffic patterns. In certain contexts, such as high-speed corridors, multiple units face debates over potential obsolescence, with some advocates favoring dedicated locomotives for better adaptability to evolving and needs, as seen in ongoing discussions within India's rail modernization efforts during the 2020s.

Global usage

Europe

In Europe, multiple units form the backbone of passenger rail services, with the operating one of the continent's largest fleets of electric multiple units (EMUs) and diesel multiple units (DMUs). introduced 25 six-car Class 717 EMUs in 2019 for Thameslink's Great Northern suburban services from to destinations like and , enhancing capacity and reliability on busy commuter routes. In , SNCF's fleet—comprising high-speed EMUs capable of 320 km/h—dominates intercity and international services, operating across a dedicated network exceeding 2,800 km of high-speed lines and serving over 100 million passengers annually. Regional variations highlight adaptations to diverse terrains and electrification levels. Germany's and private operators rely on DMUs (previously known as Talent) for non-electrified regional lines, with over 500 units in service since 2000, offering speeds up to 140 km/h and low-floor accessibility for rural connectivity. Sweden's SJ operates X2000 tilting EMUs, which achieve 200 km/h on curved conventional tracks through active tilt technology, reducing travel times on routes like to by up to 30 minutes compared to non-tilting trains. In , SNCB is procuring up to 442 CAF hybrid battery-electric multiple units (BEMUs) under a €1.7 billion framework to support cross-border services, enabling seamless operation across DC and AC electrification zones into and the . Recent developments underscore a continent-wide shift toward sustainable operations. The is advancing , with 57.4% of its 201,000 km rail network electrified by 2023—up from 53.8% in 2013—and carrying 80% of rail traffic on these lines, aligning with goals for further expansion to support net-zero emissions by 2050. Ireland's DART+ program includes deploying up to 750 carriages, forming new EMUs and BEMUs from 2025 to extend electrified services 40 km westward and increase frequencies to 15 trains per hour on key commuter lines. Switzerland's SBB has introduced Stadler RABe 501 Giruno EMUs since 2019, optimized for alpine routes via the 57 km , achieving 250 km/h and facilitating international with and . Europe's multiple unit operations span over 94,000 km of rail lines within the (TEN-T), where Technical Specifications for Interoperability (TSIs) ensure standardized loading gauges, braking systems, and ERTMS signaling for seamless cross-border deployment. This framework supports the predominance of EMUs on electrified core corridors, enhancing efficiency across the EU's integrated passenger network.

Asia

Asia's adoption of multiple units (MUs) has been marked by pioneering innovations and extensive urban networks to accommodate dense populations. Japan leads in high-speed applications with the , which has exclusively utilized electric multiple units since its inauguration in 1964 as the world's first line, initially operating at speeds up to 210 km/h. Subsequent advancements have pushed operational speeds beyond 300 km/h, with models like the N700 series achieving up to 320 km/h on dedicated tracks, emphasizing reliability and in a seismically active region. has rapidly expanded its high-speed MU fleet, particularly with the CR400 Fuxing series, which operates at 350 km/h and forms one of the world's largest deployments, exceeding 1,100 trainsets by 2025 to support a network spanning over 48,000 km. Urban and regional MU services in Asia address massive commuter demands through efficient electric multiple units (EMUs). In , the Korea Train Express (KTX) system employs EMUs like the , a domestically developed model reaching 260 km/h for intercity travel, while Seoul's subway network relies on EMUs for high-frequency urban service, carrying millions daily with advanced . India's Mumbai Suburban Railway operates one of the world's busiest networks, with over 2,300 daily EMU services transporting more than 7.5 million passengers across 450 km of tracks, featuring 12- to 15-car rakes designed for peak-hour intensity. In , (KAI) deploys EMUs on Java's commuter lines, including the in Greater Jakarta, where new Chinese-built sets enhance capacity on routes serving over 1 million daily riders with features like air-conditioning and . Emerging markets in are integrating MUs to modernize rail infrastructure, often blending diesel and hybrid technologies for varied terrains. The Philippines' (PNR) operates diesel multiple units (DMUs) such as the sets introduced in 2009 and INKA-built units since 2019, providing commuter services along the line with capacities for up to 300 passengers per car to alleviate urban congestion. In , the is procuring 184 hybrid diesel-electric multiple units (Hybrid DEMUs) as part of a 24.1 billion baht modernization plan, aimed at rural and routes to improve access and reduce emissions in underserved areas. Asian MU operations face unique challenges, particularly overcrowding and natural hazards, prompting innovative safety measures. In , designs incorporate seismic-resistant features, such as base isolation systems and earthquake early-warning integration, ensuring zero passenger fatalities since 1964 despite frequent tremors. To combat overcrowding on dense urban lines, widespread installation of (PSDs) has become standard, with over 4,000 platforms targeted for equipping by 2030 to prevent falls and enhance airflow, significantly reducing accidents in high-traffic stations.

Africa and Middle East

In and the , multiple units have been adopted to address diverse challenges in expanding rail networks, including urban congestion, freight demands in arid environments, and connectivity in developing . 's Passenger Rail Agency of (PRASA) operates a significant fleet of electric multiple units (EMUs) for its commuter services, with a major procurement of 600 six-car EMU sets—totaling 3,600 cars—under a R51 billion contract signed with in 2013 to modernize urban and suburban routes around and . These units, known as X'trapolis Mega, feature advanced communications and have been progressively introduced since 2017 to replace aging and improve reliability on electrified lines. Meanwhile, the system in province utilizes 24 four-car EMU sets supplied by Bombardier (now ) since its 2010 launch, serving high-capacity airport and urban links with a top speed of 160 km/h. In North Africa, Algeria has integrated diesel multiple units (DMUs) into its rail system during the 2010s to enhance urban and regional connectivity, particularly for shorter routes where electrification is limited. The first such DMUs were deployed by the Société Nationale des Transports Ferroviaires (SNTF) to support extensions of the national network, including links to Algiers' growing suburbs, as part of broader urban rail projects that also encompass the Algiers Metro's EMUs. In Egypt, the Cairo Metro and associated light rail systems rely on Chinese-manufactured EMUs to alleviate transport pressures in the densely populated capital. CRRC Sifang supplied the country's first urban EMUs in 2018 under a contract for six-car sets with a maximum speed of 120 km/h, which entered trial operations in 2022 on the interurban line from Cairo to 6th of October City and extensions to satellite cities like New Cairo, accommodating up to 2,222 passengers per train. These units incorporate energy-efficient regenerative braking and are maintained under a 12-year agreement to ensure operational continuity. The has seen the introduction of freight-oriented multiple units to bolster logistics in harsh desert conditions. In the , launched its national freight network in February 2023, spanning 900 km and utilizing diesel-electric multiple units alongside heavy-haul locomotives to transport over 60 million tonnes annually, connecting ports like to industrial hubs in and beyond. This expansion includes specialized designs for sand-prone routes, with the full passenger DMU fleet—21 units from —set for integration by 2026 to complement freight services. Operational challenges in these regions necessitate adaptations for extreme environments, particularly desert-resistant features in multiple units. Sand accumulation on tracks and equipment is mitigated through innovative designs like curved sand deflectors installed along rights-of-way and self-cleaning air intake filters on locomotives and power cars, as applied in Middle Eastern networks such as Saudi Arabia's North-South line and UAE's . In , hybrid diesel-electric multiple units address non-electrified lines, with Kenya Railways deploying DMUs for regional services on extensions of the (SGR), including training programs completed in 2025 to operate these self-propelled units on diesel-powered segments from toward and Malaba. These hybrids provide flexibility for mixed-traffic routes amid ongoing debates. Recent growth trends underscore investment in high-speed multiple units for enhanced regional connectivity. In , the high-speed line—Africa's first, operational since 2018 between and —benefits from 2025 allocations of approximately $3.1 billion in national transport investments, including extensions totaling 430 km to , Marrakech, and other cities using TGV-derived sets capable of 320 km/h. This funding, part of a broader 28 billion dirham infrastructure plan, aims to support events like the and boost economic integration across .

North America

In , multiple units (MUs) are increasingly adopted in systems to enhance efficiency and reduce emissions, particularly in urban corridors facing aging infrastructure. In the United States, Southern California's Metrolink has pursued conversions to zero-emission multiple units (ZEMUs) through a partnership with Los Angeles Metro, funded by a grant to transform existing rail cars into battery-electric MUs for non-electrified routes, with initial pilots in the early 2020s aimed at integrating sustainable propulsion without full electrification. Similarly, Chicago's has ordered eight two-car battery-electric multiple units (BEMUs) from in 2024, valued at $154 million, with options for additional sets to serve low-volume branches and replace diesel locomotives, emphasizing compact designs that seat 112 passengers per set while complying with (FRA) safety standards. In , , operated by in the , is modernizing its bi-level fleet with mid-life overhauls of 181 cars by starting in 2026, incorporating upgrades for future bi-mode operations that blend diesel and electric capabilities to support electrification plans along key corridors. For intercity and freight applications, MUs remain limited but are evolving with high-profile upgrades. Amtrak's , serving the , introduced NextGen trainsets—electric multiple units built by as part of the platform—in August 2025, following delays from an initial 2023 target; these 28-trainset units achieve top speeds of 160 mph, offer 25% more than predecessors, and feature advanced amenities like ergonomic seating and onboard to boost ridership on the busiest U.S. passenger route. Freight MUs are experimental and scarce due to the dominance of locomotive-hauled consists, but has tested hybrid configurations, including a 2020s trial pairing an experimental battery locomotive with diesel units on a Barstow-to-Stockton route, demonstrating potential for reduced fuel use in distributed power setups without full MU integration. Regulatory frameworks shape MU deployment, with the FRA enforcing stringent crashworthiness standards under 49 CFR Part 238 to protect passengers and crew in collisions, including requirements for anti-climber devices, crash energy management, and occupant protection in Tier equipment—standards that have driven designs like those in Metra's BEMUs and Acela's NextGen sets. These rules, updated in 2023 to incorporate alternatives for high-speed trainsets, prioritize structural integrity over lighter European-style MUs, influencing North American adaptations for shared freight-passenger tracks. In , the Tren Maya project launched diesel multiple units (DMUs) from Alstom's family in December 2023, with full 1,554 km circuit operations by December 2024; these 42 Mexican-built trains, operating at up to 160 km/h, connect tourist sites across the while adhering to local safety norms derived from international standards. Emerging trends highlight electrification pilots amid widespread infrastructure challenges, such as deferred maintenance on legacy diesel lines. California's completed its $2.4 billion project in 2024, deploying 19 Siemens Mireo Plus B EMUs for starting in fall 2024, which have driven a 78% ridership surge by July 2025 through faster acceleration and quieter operations on the San Francisco-to-San Jose corridor, serving as a model for transitioning from aging diesel fleets to sustainable MUs. These initiatives reflect a broader push for MUs to address capacity constraints and environmental goals, though regulatory hurdles and high costs limit widespread adoption compared to locomotive-dominated networks.

Oceania and South America

In , multiple units play a key role in addressing the region's vast distances and varied terrain, particularly in where electric multiple units (EMUs) dominate urban and intercity services. operates the Tilt Train, a tilting (DMU) service introduced in 2003, capable of speeds up to 160 km/h on the to route, enhancing connectivity across the state's electrified and non-electrified networks. The electric variant, built by , functions as an EMU and supports sustainable travel on the same corridor, reducing reliance on traditional locomotives. In , ' Waratah series represents one of Australia's largest EMU fleets, with 78 eight-car A sets and additional B sets totaling over 600 carriages, designed for high-capacity suburban operations since 2011. These double-decker units, manufactured by a including , prioritize passenger comfort and efficiency on the densely populated network. New Zealand's adoption of multiple units remains limited, focusing on regional and scenic routes where diesel multiple units provide flexible service amid challenging geography and low population densities. The ADL/ADC class DMUs, originally acquired from in the 1990s, served 's suburban network until their retirement in 2022, offering a cost-effective alternative to locomotive-hauled trains on non-electrified lines. For scenic operations, such as those on the Main North Line, DMUs like the ADK/ADB class have been used sporadically to navigate routes with tight curves and gradients, though most tourist services rely on hauled consists. Adaptations for New Zealand's seismic activity include reinforced underframes and suspension systems in EMUs on the electrified and lines, ensuring operational continuity during earthquakes by minimizing risks. In South America, multiple units are integral to urban mass transit and freight in seismically active and tropical environments, with Brazil's Companhia Paulista de Trens Metropolitanos (CPTM) operating one of the continent's largest EMU fleets in São Paulo. The CPTM network features over 1,000 carriages across various series, including the Series 7000 with 80 eight-car sets totaling 640 cars, delivered between 2009 and 2011 to handle peak-hour demands on seven lines serving 95 stations. These stainless-steel EMUs incorporate tropical climate modifications, such as enhanced anti-corrosion coatings on underbodies and electrical components to combat high humidity and salt exposure in coastal areas. In Argentina, freight multiple units are emerging to modernize logistics, with CRRC supplying 50 DMUs in 2023—the largest such order in the country's history—for Trenes Argentinos operations, enabling efficient short-haul mineral transport on broad-gauge lines without full locomotive attachments. Chile's Metro de Santiago expansions emphasize resilient EMUs tailored to frequent earthquakes, with the network incorporating over 300 new cars since 2017 for Lines 3, 6, and extensions on Lines 2 and 4. Alstom's series trains, deployed on these lines, feature earthquake-resistant designs including flexible bogies and systems that maintain stability during seismic events up to magnitude 8.8, as tested in the 2010 Maule earthquake. Trenes de Chile has ordered 32 additional EMUs from for commuter extensions, including the 61 km Santiago-Melipilla line, with deliveries starting in 2026 to boost capacity amid urban growth. These units include base isolation mounts to absorb shocks, drawing from 's advanced seismic engineering standards. Recent projects highlight expanding multiple unit deployments. In , the Metro's Line 2, which began partial operations in 2023 and continues expanding into 2025, has introduced initial six-car EMUs from to serve the 27 km east-west corridor connecting up to 1.5 million daily passengers with earthquake-dampening suspensions adapted for the Andean region. In , proposals for mining freight multiple units are advancing under the 2023 National Railway Plan, targeting transport from remote highlands to ports via lightweight DMUs on a proposed 300 km network, though full implementation awaits funding.

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  141. https://www.mdpi.com/2075-4701/13/5/995
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  143. https://www.pandrol.com/us/casestudies/santiago-metro/
  144. https://www.railwaygazette.com/traction-and-rolling-stock/efe-orders-32-emus-for-santiago-suburban-routes/64727.article
  145. https://www.pandrol.com/insight/engineering-a-solution-in-an-earthquake-zone/
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  148. https://www.railwaygazette.com/infrastructure/papua-new-guinea-railway-proposal/63352.article
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