thumb|EMD TR1 diesel locomotive with two units—cow and calf
In rail transport, a cow–calf (also cow and calf, or in the 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.
Distinctions between cow–calf, B units, and slugs
Calves are similar to slugs (cut-down locomotives which do not have their own engines, but may have control cabs) and especially B units (powered booster locomotives which do not have cabs).
A slug is semi-permanently paired with a cabbed unit, but does not have its own engine. At low speeds, many diesel–electric locomotives can generate more electrical current than can be used by their motors. Slugs use this excess current to power their traction motors. In contrast, all units in a cow–calf set have their own engines.
The Union Pacific Railroad made use of cow–calf sets as helpers on a steep grade near Kelso, California, until 1959, when the use of multiple-unit train control made them obsolete. Most American examples were replaced by road switcher locomotives. The Belt Railway of Chicago was the final holdout, continuing to operate TR2 and TR4 sets into the 1980s and 1990s.
- EMD TR
- EMD TR1
- EMD TR2
- EMD TR3
- EMD TR4
- EMD TR5
- EMD TR6
Other cow–calf models
The American Locomotive Company (ALCO) built two cow–calf sets, derived from the ALCO S-6 and designated SSB-9.
Baldwin Locomotive Works produced nine cow–calf versions of the Baldwin S-8. Both ALCO and Baldwin's cow–calf sets all went to customer Oliver Mining. British Steel created some in a similar manner in 1971 for use at Port Talbot Steelworks. Four locomotives built by Brush Traction between 1954 and 1957 had their cabs removed and control gear moved into metal cabinets as slaves. Five similar locomotives were equipped to work as masters. The conversions were done in 1971 and they were taken out of service in 1986.