A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as multiple units, motor coaches, railcars or power cars; the use of these self-propelled vehicles is increasingly common for passenger trains, but rare for freight (see CargoSprinter and Iron Highway).
Traditionally, locomotives pulled trains from the front. However, push-pull operation has become common, where the train may have a locomotive (or locomotives) at the front, at the rear, or at each end.