multiple unit
A train consisting of one or more self-propelled cars capable of coupling with other units of the same or similar type and still being controlled from one cab.
Multiple unit: train cars that drive themselves and work together
A multiple unit is a train made up of two or more self-propelled passenger or freight cars that can be coupled together and operated as a single train from a single driver's cab. Unlike a locomotive-hauled train, where one powered engine pulls unpowered cars behind it, every car in a multiple unit set carries its own traction motors. This distributed power arrangement allows shorter acceleration times, better weight distribution on the track, and the ability to operate profitably on routes with frequent stops and lower passenger volumes.
The fundamental requirement is that coupled units must be able to transmit driving commands from one cab to all other units in the consist. Historically this was done through pneumatic or mechanical linkages; modern multiple units use electrical or electronic control systems. A typical urban multiple unit might consist of four to nine cars, though longer sets exist on high-capacity metro systems. The cars are permanently or semi-permanently coupled, and each powered car normally has the same number and type of traction motors, ensuring balanced power delivery.
Common configurations and applications
Multiple units dominate suburban and urban rail networks where frequent acceleration and deceleration are necessary. British Rail Class 317 electric units, for example, consist of four cars with motors on two of them, achieving 100 mph capability on regional routes. Driverless automatic people movers often use very short multiple unit sets of two or three cars. Freight multiple units are less common but do exist, particularly in Europe for wagonload and block train operation. Some railway operators couple multiple unit sets together to form longer trains during peak hours, requiring compatible electrical or pneumatic control interfaces.
The power rating of a multiple unit is distributed across several motorised cars rather than concentrated in one locomotive. A four-car unit might have 800 kW total installed power spread across two motor cars, compared to a single 1600 kW locomotive hauling the same consist. This affects climbing ability, maximum speed, and acceleration performance. Regenerative braking, where motors act as generators during deceleration, is easier to implement in multiple units and recovers energy on electrified lines.
Maintenance of multiple units requires competence across multiple powered cars and their control systems. Each motor car needs regular inspection of its traction motors, suspension, and braking equipment. Control system failures that prevent cab communication across the consist render the entire train unusable, even if individual motor cars are mechanically sound. This is why multiple unit fleets typically require depot facilities with capacity to test inter-unit electrical and control connections, not just individual car mechanics.