electric multiple unit
A multiple unit train powered by electricity, abbreviated EMU.
EMU: a train where every car pulls its own weight
An electric multiple unit is a train in which power is distributed to multiple cars rather than concentrated in a single locomotive. Each powered car carries its own traction motor and draws current from the overhead catenary or third rail, so the train has no separate engine. The result is lighter weight per seat, faster acceleration, and better weight distribution along the track compared to a locomotive-hauled train of the same capacity.
EMUs come in rigid or articulated configurations. Rigid units are independent cars coupled together; articulated units are permanently joined at bogies (wheel trucks), reducing gaps and dead weight. A typical modern regional EMU might have four to ten cars, with power cars at one or both ends and unpowered trailer cars in between. Some designs distribute motors across all cars; others concentrate them in specific cars. The power-to-weight ratio typically ranges from 10 to 20 kilowatts per tonne, allowing acceleration of 0.8 to 1.2 metres per second squared from a standstill.
Current collection is the critical interface. Overhead catenary systems require a pantograph on the roof; third-rail systems (used in some metros and older commuter networks) use a sliding shoe contacting a live rail at platform level. Most modern EMUs use ac catenary at 15 kV or 25 kV; some older systems and metros use dc at 600 to 1500 volts. The onboard transformer and rectifier convert this to the voltage and waveform needed by the traction motors, almost always three-phase ac induction motors or synchronous permanent-magnet motors today.
Why EMU, not locomotive-hauled?
EMUs dominate commuter and regional rail because they accelerate faster, stop more responsively, and use less energy per passenger-kilometre than conventional trains. They also occupy less depot space. The drawbacks are higher manufacturing complexity, limited flexibility in reconfiguring trainsets, and greater difficulty in emergency towing. An EMU cannot easily be separated into smaller units or pushed by another train if its power system fails.
The term originated in Britain in the 1930s and is now standard in European and Australian rail engineering. North American practice often uses the term multiple unit more broadly to include both electric and diesel-electric distributed-power trains. In Japan and parts of Asia, almost all passenger rail uses EMU technology. Urban metros, light rail, and commuter networks are now EMU-first; main-line freight and long-distance passenger services continue to use conventional locomotives, though some railways are experimenting with EMU trainsets for overnight and express services.