Multi-system (rail)

A multi-system locomotive, also known as a multi-system electric locomotive, multi-system electric multiple unit, or multi-system train, is an electric locomotive which can operate using more than one railway electrification system. Multi-system trains provide continuous journeys over routes which are electrified using more than one system.

A multi-system locomotive is useful for providing a single journey over multiple electrification systems without interruption, either from requiring passengers to change trains or from changing locomotives. Such conditions can occur where a train crosses national boundaries and each country has implemented a different electrification system.

Prior to 1945 there was no demand for multi-system locomotives in Europe. From the 1950s onwards, the emerging formation of the European Union, and the consequent increase in the amount of cross border traffic, along with the addition of a 25 kV 50 Hz AC system in France in addition to the older 1.5 kV DC system, gave rise to the need for multi-voltage locomotives. Very high capital costs prevent or hinder the adoption of a standard railway electrification system.

At the beginning of the 21st century, railway legislation in Europe (the First Railway Package and Second Railway Package, and the creation of a Trans European Rail Freight Network) liberalised cross border freight traffic, giving rise to a demand for locomotives that could work between European Union countries with different electrification systems. That created a practically new market for multi-voltage locomotives, such as Bombardier's TRAXX. However, the increase in the cost of locomotives and maintenance, along with the expense of installing different safety systems for cross-border work, reduced the economic viability of multi-system vehicles versus the use of single-voltage machines or changing locomotives where electrical systems change.

South Africa has 15 km (9.3 miles) of dual system track, both 3 kV DC and 25 kV AC.^{[citation needed]}

Sections of track between Cheongnyangni station and Hoegi station, as well as between Seoul Station and Namyeong station on Seoul Subway Line 1, and the section of track (including the flying crossover) between Namtaeryeong station and Seonbawi station on Seoul Subway Line 4, are dual system-equipped with the subway standard 1500 V DC and mainline railway standard 25 kV 60 Hz AC overhead line systems.

Electrification in the UK began in a piecemeal fashion. The earliest main line (as opposed to metro and tramway) systems were divided between low voltage third rail (commonly about 600 V DC) and overhead systems (a variety of voltages, both DC and AC were used). The third rail systems of this period eventually gave rise to the 750 V DC system in the southern part of the UK and a separate area with the same system around Merseyside.

Cheap loans to stimulate economic development in the 1930s gave rise to the several schemes of 1.5 kV DC electrification, mostly completed post war, notably between Liverpool Street and Shenfield, and the Woodhead Line. Starting with the West Coast Main Line electrification in the 1960s, the 25 kV AC overhead system was adopted for all subsequent mainline electrification in the UK (except for extensions to other existing systems, mostly on the southern third rail network).