Design
Most MUs are powered either by traction motors, receiving their power through a live rail or overhead wire (EMU), or by a diesel engine driving the wheels through a gearbox or hydraulic transmission (DMU). Diesel-electric multiple units (DEMUs) have a diesel engine that drives a generator producing electricity to drive traction motors in a similar fashion to a diesel-electric locomotive.
A multiple-unit trainset has the same power and traction components as a locomotive, but instead of the components being concentrated in one carbody, they are spread out on each car that makes up the set. In many cases these cars can only propel themselves when they are part of the set, so they are semi-permanently coupled. For example, a DMU might have one car carry the prime mover and traction motors, and another the engine for head end power generation; an EMU might have one car carry the pantograph and transformer, and another car carry the traction motors.
It is not necessary for every single car in an MU set to be motorized. Therefore MU cars can be motor units or trailer units. Instead of motors, trailing units can contain supplementary equipment such as air compressors, batteries, etc.; trailer cars may also be fitted with a driving cab.
In most cases, MU trains can only be driven/controlled from dedicated cab cars. However, in some MU trains, every car is equipped with a driving console, and other controls necessary to operate the train, therefore every car can be used as a cab car whether it is motorised or not, if on the end of the train. An example of this arrangement is the NJ Transit Arrows.
Read more about this topic: Multiple Unit
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