Illinois Central 9251 EMD TR1 diesel locomotive with two units—cow and calf, 1942.

(Library of Congress, via Wikimedia Commons)



In rail transport, a cow-calf (also cow and calf) is a set of diesel switcher 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, and Baldwin Locomotive Works. Cow-calf sets were made obsolete by the development of road switcher locomotives, which could handle both mainline trains and switching duties.


This locomotive and slug of the Iowa Interstate Railroad resemble a cow-calf set, but the slug has no engines of its own; this allows it to be cut down for better visibility.

(David Wilson, CC BY 2.0 <>, via Wikimedia Commons)


These EMD FP9 locomotives also resemble a cow and calf, but the B unit is designed

for mainline speeds, and to be detachable and operate with any other locomotive.

(Roger Puta, courtesy Marty Bernard from U.S.A., Public domain, via Wikimedia Commons)


Distinctions between cow-calf, B units, and slugs

Cow-calf sets 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). 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 with a drawbar 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.



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.

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.


A Belt Railway of Chicago cow-calf set in 1985.

(Roger Puta, courtesy Marty Bernard from U.S.A., Public domain, via Wikimedia Commons)


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. The Belt Railway of Chicago was the final holdout, continuing to operate TR2 and TR4 sets into the 1980s and 1990s.


EMD TR4 Cow and calf.

(Roger Puta, courtesy Marty Bernard from U.S.A., Public domain, via Wikimedia Commons)


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.



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.