An A-unit, in

railroad Rail transport (also known as train transport) is a means of transport using wheeled vehicles running in railway track, tracks, which usually consist of two parallel steel railway track, rails. Rail transport is one of the two primary means of ...

terminology, is a

diesel locomotive A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), power source is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is con ...

(or more rarely an

electric locomotive An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a Battery (electricity), battery or a supercapacitor. Locomotives with on-board fuelled prime mover (locomotive), ...

) equipped with a driving cab and a

control system A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial ...

to control other locomotives in a

multiple unit A multiple-unit train (or multiple unit (MU)) is a self-propelled train composed of one or more Coach (rail), carriages joined, and where one or more of the carriages have the means of propulsion built in. By contrast, a locomotive-hauled ...

, 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 North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

, since only there was it commonplace to build B-units—cabless locomotive units which normally could not lead a train.

Typical features

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 (an employee permitted to move a locomotive in a yard only, not on the road).

Terminology

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 A hood unit, in North American railroad terminology, is a body style for diesel and electric locomotives where the body is less than full-width for most of its length and walkways are on the outside. In contrast, a cab unit has a full-width ca ...

road switcher A road switcher locomotive is a type of railroad locomotive designed to both haul railroad car, railcars in mainline service and Shunting (rail), shunt them in railroad yards. Both type and term are North American in origin, although similar t ...

" 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''.

Conversions

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. The Chicago and North Western Railway converted several E8B units purchased from the

Union Pacific Railroad The Union Pacific Railroad is a Railroad classes, Class I freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pacific is the second largest railroad in the United Stat ...

. 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.

References

{{Reflist * Pinkepank, Jerry A. (1973). ''The Second Diesel Spotter’s Guide''. Kalmbach Books. Library of Congress Catalog Card No. 66-22894. Diesel locomotives Rail technologies