Cow-calf
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In North American railroading, a cow-calf set is a pair of switcher-type diesel locomotives: one (the cow) equipped with a driving cab, and the other (the calf) without. The two are coupled together (either with regular couplers or a semi-permanent drawbar) and are connected with MU cables and brake lines so that both locomotive units can be operated by a single cab. Cow-calf sets were used in heavy switching, hump yard switching, and transfer runs between yards.
Cows are analogous to A units and calves to B unit road locomotives. Unlike them, cow-calf sets were almost always permanently attached.
Some 3-unit cow-calf-calf sets were built, but this was rare.
Each unit in a cow-calf set was powered. As diesel engines became more powerful, they exceeded the practical limit to the power that could be applied to the rail at low speeds in a single four- or even six-axle locomotive. Thus, the concept of the slug was born—like a calf, these attached to a switcher or other locomotive needing extra low-speed tractive effort, but a slug does not have its own engine—instead, it takes electrical power from its "mother", allowing more of the power of that engine to be applied.
Most cow-calf sets were built between the 1930s and the 1950s. They were built by several different makers, although General Motors' Electro-Motive Division built far more than the others.
The cow-calf concept was adopted on Queensland 2ft gauge sugar cane railways with two locomotives being coupled permanently in multiple-unit mode with the cab of one removed. This was utilised by Isis Mill (1980-1993) and by Mackay Sugar (2005).
The concept was also used in the United Kingdom by British Rail to produce the unique Class 13 locomotives, composed of two Class 08 locomotives. British terminology is Master-and-Slave Unit.
