In rail transport, the expression power car may refer to either of two distinct types of rail vehicle:

a vehicle that propels, and commonly also controls, a passenger train or tram, often as the lead vehicle;[1][2][3]
a vehicle equipped with machinery for supplying heat or electrical power to other parts of a train.[2][3]
The first of these types of vehicle is closely related to the locomotive. What differentiates the locomotive and the first type of power car is their construction or use. A locomotive can be physically separated from its train and does nothing but provide propulsion and control (and heat or electricity for passenger trains). On the other hand, a power car of the first type is frequently an integral part of its train, and some of the car's interior space may be used for carrying passengers or cargo.

http://www.freerails.com/gallery/4158/4158_071434_260000000....

A locomotive or engine is a rail transport vehicle that provides the motive power for a train.
https://en.wikipedia.org/wiki/Locomotive