The motors driving the train on a multiple unit, are mounted underneath the floor of the carriages, on the bogies (the assembly on the underneath of a train which holds the wheels).
The driver's cab on a multiple unit is usually truncated to a short room at both ends of the train.
The advantage of multiple units, is that because they have a cab at both ends, the train can be reversed without having to uncouple\\re-couple and move the locomotive, which results in far quicker turnaround times.
Another advantage of multiple units is the possibility they afford of employing tilting technology to achieve higher speeds on older lines by hydraulically or pneumatically banking the train in curves. One of the first multiple units with tilting technology was the German class 403, nicknamed "Donald Duck".
The quicker turnaround time that results, and the reduced size compared with large locomotive-hauled trains, has made the multiple unit a major part of suburban commuter rail services in many countries. Multiple units, are also the type of train used almost exclusively on underground railways.
Often, but not always, a series of multiple units can be connected together to form a larger train. Sometimes passage is available between the units, either for passengers, or just for train crew.
Most multiple units 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.
In North America, multiple units are not common (and diesel multiple units almost non existent). However, large fleets of electric multiple units are found in some of the urban centres on the East coast of the US. North American railroaders also refer to a combination of locomotives, at the head of a train, as a multiple unit.
Some well known examples of multiple units are the German ICE and the Belgian Thalys.