An electric multiple unit (EMU) is an electric self-powered train, capable of operating in multiple with other EMUs and without the need for a locomotive; these are typically passenger trains with accommodation in every vehicle and a driving position at each end. The term can also be used to describe a train that is a permanent formation with a non-driving power car, such as the Advanced Passenger Train. As of December 2010^[update], two-thirds of the passenger carriages in Great Britain are formed in EMUs.

Electric railways began in Great Britain in 1883 and the first EMU ran on the Liverpool Overhead Railway in 1893. In the early 20th century, systems were developed where all the motors on a train could be controlled by a low voltage signal from any cab. Due to problems using steam locomotives underground, the underground railways in London and Liverpool were early adopters of 600 V DC electric traction and, by 1907, underground railways in London and some provincial cities were running electric trains. A 6.7 kV 25 Hz overhead system was also used from 1908. After the railway companies were grouped in 1923, the Southern Railway greatly expanded the third-rail DC electrification, replacing some early AC electrification. Overhead lines at 1,500 V DC were planned and installed just before and after the Second World War. The early EMU carriages, similar to contemporaneous locomotive-hauled carriages, were constructed of wood with hinged doors with accommodation in saloons or compartments. As time went on, more steel became used in carriage construction.

After the Second World War, the railways were nationalised and the 25 kV 50 Hz overhead system was installed; this eventually replaced all 1,500 V DC systems. South of London, the Southern Railway third-rail electrification system continued to expand.

Volks Electric Railway, the first electric passenger railway in Britain, opened in 1883 as a short 2-foot (610 mm) gauge electric railway running for 1⁄4 mi (400 m) in Brighton. At first electrical power at 160 V DC was supplied to a small car using the two running rails, but it was not long before a third live rail was added, a sliding shoe making contact to the top.

The City and South London Railway, a 3.2-mile-long (5.1 km) deep-level tube railway, opened in 1890 with carriages hauled by electric locomotives powered from a third rail beneath the train energised at 500 V DC. In 1893 the Liverpool Overhead Railway opened with two lightweight passenger cars coupled together; on each car one bogie was powered with a single 60 hp (45 kW) motor powered from a third rail between the tracks at 500 V DC. Any number could be coupled together and all the motors controlled by the driver at the front. In 1898, the deep-level Waterloo and City Railway opened with EMUs, a central third rail providing traction current at 500 V to 530 V DC. The current for the two motor cars could be controlled from either driving cab, but the Board of Trade became concerned about the fire risk presented by the 11 power cables carried on the roof of the train and barred installation of similar systems in deep-level tubes. Therefore, when in 1900 the Central London Railway opened its deep-level tube railway, electric locomotives hauled carriages.

In 1900, the Metropolitan Railway and the District Railway ran an experimental EMU in passenger service for six months. A four-rail system was used, two rails outside the running rails providing the traction current at 500–550 V DC. Only the traction motors in the leading motor car were used, the trailing motor coasting. In 1901 a Metropolitan and District joint electrification committee recommended a three-phase AC system with overhead wires. Initially this was accepted by both parties, but the District needed to raise finance and found an investor in 1901, the American Charles Yerkes. Yerkes soon had control of the District and his experiences in the United States led him to favour DC with a track level conductor rail pickup similar to that in use on the City & South London Railway and Central London Railway. After arbitration by the Board of Trade the DC system was taken up.

In 1897, Frank J. Sprague demonstrated a system where a low-voltage control signal from any driving position controlled all the motors on a train, developed for the South Side Elevated Railroad in Chicago. This electro-magnetic system was developed in Britain by British Thomson-Houston (BTH) and first used in Europe in 1902 on the Central London Railway when it changed over to EMU operation to solve a design problem with its locomotives. Westinghouse Electrical developed an electro-pneumatic multiple unit system that used a lower control current and when in 1903 the District opened its line from Acton Town to South Harrow with experimental EMUs these used control systems from BTH and Westinghouse. After these trials the District selected BTH equipment for its B Stock.

The underground Mersey Railway started an electric service on 3 May 1903, electrified by British Westinghouse Electric with equipment from the US. A four-rail DC system was installed, with a positive outer rail and a negative return rail between the running rails.