In rail transport, a cow-calf (also cow and calf, or in the UK master and slave) is a set of switcher-type diesel locomotives. The set usually is a pair; some three-unit sets (with two calves, also known as herds) were built, but this was rare. A cow is equipped with a cab; 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.

Cows are analogous to A units (locomotives with a cab) and calves to B unit (powered locomotives without a cab) road locomotives. The cow and calf are both equipped with prime movers for propulsion. Like the early EMD FT locomotives, the cow-calf sets were typically built as mated pairs, with the cow (or cabbed unit) and calf (or cabless unit) 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. Cow-calf locomotives can be distinguished from the sometimes very similar looking slug and slug mother sets by the fact that both cows and calves are independently powered, while slugs are engineless, and dependent on power from their "mother" units.

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 latter 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.