Cow-calf

In rail transport, a cow-calf (also cow and calf) locomotive is a set of switcher-type diesel locomotives. The set usually is a pair; some 3-unit sets (with two calves, also known as herds) were built, but this was rare. A cow is equipped with a driving cab; a calf is not. 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 from the single cab.

Cows are analogous to A units and calves to B unit road locomotives. Both have prime movers. 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, with over time many of the sets being 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.

Distinctions between cow-calf, B units, and slugs

Cow-calf sets are similar to slugs and especially B units. They differ from both in that a B unit is designed to operate with any other locomotives, while cow-calf sets are meant to be semi-permanently coupled to each other and operated together, though some cow-calf sets used standard couplers instead.

A slug is semi-permanently paired with a cabbed unit, but does not have its own engine. At low speeds, many diesel-electric locomotives 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.

History

Design and nomenclature

In a cow-calf set, the cow referred to the locomotive equipped with a cab, while calves lacked a cab. Cow-calf sets with two calves are known as "herds"; the only example of these were two TR3 series sets ordered by the Chesapeake and Ohio Railway. The cow, calf, and herd designations were nicknames and not official.

Cow-calf locomotives were designed for both transferring railroad cars between nearby classification yards in urban areas, and for switching within yards. They were built with an emphasis on tractive effort, with top speed of lesser importance.

Production and operations

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, chiefly its TR (transfer) series. In addition to the transfer duties they were designed for, cow-calf sets were also used in hump yards to send cuts of cars over the hump for classification.

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, which could both handle switching duties and haul mainline trains. The Belt Railway of Chicago was the final holdout, continuing to operate TR2 and TR4 sets into the 1980s and 1990s.

List of cow-calf models

TR series

EMD's TR (transfer) series were the largest group of cow-calf locomotives built. Produced in seven models, eighty were built between 1940 and 1953, along with two additional calves.
* 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.

Three pairs of cow-calf locomotives were built in the United Kingdom in the 1960s by combining two British Rail Class 08 locomotives together, these were designated the British Rail Class 13.

See also

* Slug (railroad)
* Twin unit

References

{{DEFAULTSORT:Cow-Calf

Diesel locomotives