An Electric Multiple Unit (EMU) is a type trainset of that powers itself. The earliest, therefore most basic EMUs, were passenger carriages (rollingstock) that had traction (electric) motors added to them. The type gets its name from the multiple powered cars that create it. Most EMUs have at least four cars or more, and many have traction motors on each car. One of the best features of EMUs is that they have a high power to weight ratio, meaning that EMUs are perfect for commuter lines. Which need rapid acceleration between stops to keep these difficult lines on scheduled. Underground metro EMUs usually use third-rail electrical systems, above-ground EMUs mostly use pantographs.
The EMU originates from around 1900. In Britain, the City & South London Railway has been using EMUs since it opened with 450 volt DC third-rail sets in 1890. The London Underground features the use of EMUs, as do most other underground metro lines.
EMUs were first used because of their low maintenance costs, and electric coupling. This means that one driver is able to control a whole train. Seems more impressive if one thinks that each car is powered, and this is at the time of steam locomotives. For fifty years, since around 1900, there wasn't much change in the design. But in the 1950s, new technology, pushed past the boundaries and reinvented the whole design of EMUs. Early EMUs were DC (direct current), but since the late-1950s AC (alternating current) engines have been used. Most third-rail systems are still low-voltage DC systems.
EMUs are very important in Europe. The Netherlands coined the phrase "Sprinter" (in a railway sense) to mean a two-car unit. British Rail was quick to adopt this themselves.
There are two main types of Electric Multiple Units; those with power-cars, and those with traction motors throughout the set. The latter is designed to lower the axle load and/or to give better traction. While power-car versions have traction motors on the axles of two or more cars out of the set. Non-powered cars reside between the power cars which are usually at each end.
- Book: The Complete Book of Locomotives, written by Colin Garratt & published by Hermes House. ISBN: 978-1-84477-022-9.