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AnA-unit, inrailroad terminology, is adiesel locomotive (or more rarely anelectric locomotive) equipped with a drivingcab and acontrol system to control other locomotives in amultiple unit, and therefore able to be the lead unit in aconsist of several locomotives controlled from a single position.[1] This terminology is generally used inNorth America, since only there was it commonplace to buildB-units—cabless locomotive units which normally could not lead a train.
Typical driving cab features, and therefore A-unit features, include windshields, rectangular side windows, crew seats, heating, and sometimes, radios, air conditioning and toilets. B-units always lack all of these features, except that someEMD F-units have an extra porthole-style side window(s) for ahostler (an employee permitted to move a locomotive in a yard only, not on the road).
This terminology has fallen out of use for newer locomotives, since it only really applied to thecab unit style of locomotive. Thus, the termcab unit is used only when an A-unit has a carbody design.Hood unit "road switcher" types were generally equipped with driving cabs and the term "A-unit" was not generally applied to them, although the rare cabless road switchers were still calledB-units.
In some cases, A-units were converted to B-units. If the unit had been involved in a collision which damaged the cab, it was sometimes more cost-effective to rebuild the unit without the cab. In rarer cases, B-units were converted to A-units. TheChicago and North Western Railway converted severalE8B units purchased from theUnion Pacific Railroad. The cabs on the rebuilt units were referred to as "Crandall Cabs." The BNSF also experimented with a single GP60B to make it an A-unit by using an Ex-UP SD40-2 cab on a GP60B frame and body, also required to move was the Dynamic blister from the front of the unit to the middle of the unit to make room for the cab.