A multiple unit is a passenger train whose carriages have their own motors, either diesel ("DMUs") or electric ("EMUs"), and do not need to be hauled by a locomotive.

History and description

Multiple units (MUs) were made possible by the development of multiple-unit train control by the American inventor (Franklin J. Sprague), originally to allow newly electrically-powered rapid transit trains to be operated from a single position without the need for a separate locomotive, as was required when such trains were hauled by steam engines.

Sprague solved the problem of operating all of the train's motors simultaneously from a single position. Before his successful invention, differences in the speed and response of motors on different cars of the train caused binding on the couplings between the train cars, wheel slippage and excess wear on motors and operating gear running at speeds faster or slower than the overall speed of the train, or even derailment, as well as an incomfortable ride.

The motors driving the train on an MU unit are typically mounted underneath the floor of the carriages, on the bogies (in the U.S. "trucks"), the assembly beneath the train that holds the axles and wheels.

The driver's cab on an MU is usually truncated to a short room at both ends of the train.

Advantages of multiple units

There are several advantages of multiple units as compared to locomotive-hauled trains.

• MUs are more energy efficient than locomotive-hauled trains and more nimble, especially on grades, as much more of the entire train's weight (sometimes all of it) is placed on power-driven wheels, rather than suffer the dead weight of unpowered coaches;

• MUs have cabs at each end, so that the train may be reversed without having to uncouple/re-couple and move the locomotive, which results in far quicker turnaround times, reduced crewing costs, and enhancing safety;

• MUs may usually be quickly made up or broken up into trains of varying lengths. In a handful of applications, several multiple units may run as a single train, then be broken at a junction point into smaller trains for diverse destinations. Sometimes passage is available between the units, either for passengers or just for the train crew.

The quicker turnaround time that results, and the reduced size compared with large locomotive-hauled trains, has made the MU a major part of suburban commuter rail services in many countries. MUs are also the type of train used almost exclusively on underground railways.

Most MUs are powered either by a diesel engine driving the wheels through a gearbox (a diesel multiple unit, or DMU), or by electric motors, receiving their power through a live rail or overhead wire (an electric multiple unit or EMU). However, diesel electric multiple units (DEMUs) also exist: these have a diesel engine which drives a generator producing electricity to drive electric motors.

Some well known examples of multiple units are the last generation German ICE and most Dutch passenger trains.

North America

Most long-distance trains in North America are locomotive-hauled, but commuter, subway, and light rail operations use extensive use of MUs. Most electrically-powered trains are MUs, although there are some major exceptions: Amtrak trains on the Northeast Corridor, the busiest passenger rail line in the U.S., are drawn by electric locomotives; New Jersey Transit service on the same line is split between electric locomotives and electric MUs. DMUs are less common, partly because new light rail operations are almost entirely electric, but DMUs are being tried on the River Line in New Jersey, and there are efforts to develop effective passenger DMUs for inter-city trains. NJ Transit has also experimented with DMUs on the Princeton Branch line.

See also

• Push-pull (mode of operation for locomotive-hauled trains)
• Rail terminology
• Multiple-unit train control