An A unit, in railroad terminology, is a locomotive (generally a diesel or electric locomotive) equipped with a driving cab, or crew compartment, and the control system to control other locomotives in a multiple unit, and therefore able to be the lead unit in a consist of several locomotives controlled from a single position. This terminology is generally used in North America, since only there was it commonplace to build B units—cabless locomotive units which 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 some EMD F-units have an extra porthole-style side window(s) for a hostler. A hostler is 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 the cab unit style of locomotive. Thus, the term cab 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 called B units.
Reference
- Pinkepank, Jerry A. (1973). The Second Diesel Spotter’s Guide. Kalmbach Books. Library of Congress Catalog Card No. 66-22894.
