Cow–calf

In rail transport, a cow–calf (also cow and calf, or in the UK master and slave) is a set of diesel switcher locomotives. The set is usually a pair, though a few three-unit sets (with two calves, also known as herds) were built. A cow is equipped with a cab, and a calf is not. The two are coupled together (either with regular couplers or a semi-permanent drawbar) and equipped with multiple unit train control so that both locomotives can be operated from the single cab.

A cow is analogous to an A unit, i.e. a locomotive with a cab, and a calf to a B unit, i.e. a powered, cabless road locomotive. That is, the cow and calf are each equipped with at least one prime mover for propulsion. A cow–calf set is distinct from a slug-and-mother set in that a cow and a calf are each independently powered while a slug has no prime mover and instead is dependent on power from its mother unit. Like the early EMD FT locomotives, cow–calf sets were typically built as mated pairs, with the cow and calf sharing a number. However this was not always the case, as over time many of the sets were broken up and couplers added to aid with versatility.

Most cow–calf sets were built by Electro-Motive Division (EMD), although other examples were built by the American Locomotive Company, Baldwin Locomotive Works, and British Rail (the last by combining existing locomotives together). Cow–calf sets were made obsolete by the development of road switcher locomotives, which could handle both mainline trains and switching duties.