A head end power car is a rail car that supplies head end power

Head end power

Head end power or electric train supply is a rail transport term for the electrical power distribution system on a passenger train. The power source, usually a locomotive at the front or “head” of a train or a generator car, generates all the electricity used for lighting, electrical and other...

 (HEP). Since most modern locomotives

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

 supply HEP they are mostly used by heritage railways

Heritage railway

thumb|right|the Historical [[Khyber train safari|Khyber Railway]] goes through the [[Khyber Pass]], [[Pakistan]]A heritage railway , preserved railway , tourist railway , or tourist railroad is a railway that is run as a tourist attraction, in some cases by volunteers, and...

that use older locomotives, or by railroad museums that take their equipment on excursions. Quite a few head end power cars started out as other forms of rolling stock that have been rebuilt with diesel generators and fuel tanks to supply HEP to the passenger equipment.