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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 term booster unit is also used. The concept was largely confined to North America. Elsewhere, locomotives without driving cabs were 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.
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. In the early days of dieselization, there was also no argument from the railroad unions that a B unit deserved a second crew; this was a major point of contention for a while. Additionally, at first, railroads bought multiple-unit diesel locomotives as one for one replacements for steam locomotives; the flexibility of interchangeable units which could be assembled into any power output the railroad desired was not well realised. 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 and 1950s. When hood unit road switchers became the common kind of diesel locomotive, some B units were built, but soon railroads realised that a cheaper B unit was not worth the lack of operational flexibility. Few B units have been built in the last 40 years.
List of B unit locomotive models
These are all known B unit models, with discrepancies settled by the later (Marre) reference. At least one of each model was manufactured. All units below contain one or more engines and traction motors, so cow-calf units and slugs are not listed.
- ALCO/MLW - Black Maria Booster, DL-110, FB-1, FB-2, FPB-2, FPB-4, PB-1, PB-2, C855B, M420B
- Baldwin - AS-616B, DRS-6-6-15B, DR-4-4-15B, RF-16B, DR-6-4-15B, DR-6-4-20B
- EMD E units - EB, E1B, E2B, E3B, E4B, E5B, E6B, E7B, E8B, E9B
- EMD F-units - FTB, F2B, F3B, F7B, F9B
- EMD Hood units – DD35, GP7B, GP9B, GP30B, GP60B, SD24B, SD40B, SD40-2B, SD42B
- Fairbanks-Morse - B Erie, CFB-16-4, CFB-20-4, CPB-16-4, CPB-16-5
References
- Marre, Louis A. (1995). Diesel Locomotives: The First 50 Years. Kalmbach Publishing Co. ISBN 0-89024-258-5.
- Pinkepank, Jerry A. (1973). The Second Diesel Spotter's Guide. Kalmbach Books. Library of Congress Catalog Card No. 66-22894.
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