B unit

A B unit, in railroad terminology, is a locomotive unit (generally a diesel locomotive) which does not have a driving cab or crew compartment, and must therefore be controlled from another, coupled locomotive with a driving cab (an A unit). The terms booster unit and cabless are also used. The concept is largely confined to North America. Elsewhere, locomotives without driving cabs are rare.

Some B units cannot be moved without a controlling unit attached, but most have some simple controls inside, and often a side window at that control station. For example, B unit versions of the EMD FT with conventional couplers had a fifth porthole-style window added on the right side only for the control station. Other models used existing windows. These controls enable a hostler to move the B-unit locomotive by itself in a yard or shops. (A hostler is an employee permitted to move locomotives within the confines of a yard or shops complex, but not on the main railroad.) B units without controls are generally semi-permanently coupled to controlling units. Sometimes, there is a terminology distinction between the types: a booster is a B unit with hostler controls, and a slave is a B unit without hostler controls.

The reasons railroads ordered B units included the fact that a B unit was slightly cheaper. With no driving cab, B units lack windshields, crew seats, radios, heating, and air conditioning. There would also be no toilets, which were usually found in the short hood of an A unit. Additionally, at first, railroads bought multiple-unit diesel locomotives as one-for-one replacements for steam locomotives; as a result, railroads could not take advantage of the flexibility afforded by interchangeable units, which could be assembled into any required power output. When a three- or four-unit locomotive was considered an indivisible unit, there was no point in the intermediate units having cabs. Finally, B units gave a smoother line to the train for passenger service.

B units were commonly built in the cab unit days in the 1930s, 1940s, 1950s and 1960s. When hood unit road switchers became the common kind of diesel locomotive, some B units were built, but many railroads soon came to the opinion that the lower cost of a B unit did not offset the lack of operational flexibility. Few B units have been built in the last 40 years. Railroads that kept ordering B units longer than most were largely Western roads, including the Southern Pacific, Union Pacific, Burlington Northern, and the Santa Fe. Santa Fe ordered the GP60B model in 1991, which were the final B units built for road service in North America as of 2005.

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