An A unit, in railroad terminology, is a locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

 (generally a diesel

Diesel locomotive

A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...

 or electric locomotive

Electric locomotive

An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device...

) equipped with a driving cab

Cab (locomotive)

The cab, crew compartment or driver's compartment of a locomotive is the part of the locomotive housing the train driver or engineer, the fireman or driver's assistant , and the controls necessary for the locomotive's operation....

, or crew compartment, and the control system

Multiple-unit train control

Multiple-unit train control, sometimes abbreviated to multiple-unit or MU, is a method of simultaneously controlling all the traction equipment in a train from a single location, whether it is a Multiple unit comprising a number of self-powered passenger cars or a set of locomotives.A set of...

 to control other locomotives in a multiple unit

Multiple unit

The term multiple unit or MU is used to describe a self-propelled carriages capable of coupling with other units of the same or similar type and still being controlled from one driving cab. The term is commonly used to denote passenger trainsets consisting of more than one carriage...

, and therefore able to be the lead unit in a consist

Consist

A consist , in North American railway terminology, is used as a noun to describe the group of rail vehicles that make up a train. A near-equivalent UK term is rake but this excludes the locomotive....

 of several locomotives controlled from a single position. This terminology is generally used in North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, since only there was it commonplace to build B unit

B unit

A "B" unit, in railroad terminology, is a locomotive unit which does not have a driving cab, or crew compartment, and must therefore be controlled from another, coupled locomotive with a driving cab . The term booster unit is also used. The concept was largely confined to North America...

s—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-unit

EMD F-unit

EMD F-units were a line of Diesel-electric locomotives produced between November 1939 and November 1960 by General Motors Electro-Motive Division and General Motors-Diesel Division. Final assembly for all F-units was at the GM-EMD plant at La Grange, Illinois and the GMDD plant in London, Ontario...

s have an extra porthole-style side window(s) for a hostler

Hostler

An hostler or ostler in the horse industry is a groom or stableman, who is employed in a stable to take care of horses...

 (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

Cab unit

A cab unit and a carbody unit are body styles of locomotives in railroad terminology. While closely related, they are not exactly the same....

 style of locomotive. Thus, the term cab unit is used only when an A unit has a carbody

Cab unit

A cab unit and a carbody unit are body styles of locomotives in railroad terminology. While closely related, they are not exactly the same....

 design. Hood unit

Hood unit

A hood unit, in railroad terminology, is a body style for diesel and electric locomotives. On a hood unit, the body of the locomotive is less than full-width for most of the locomotive's length, with walkways on the outside of the locomotive. In contrast, a cab unit has a full-width carbody for...

 "road switcher

Road switcher

A road switcher is a type of railroad locomotive used for delivering or picking up cars outside of a railroad yard. Since the road switcher must work some distance away from a yard, it needs to be able to operate at road speeds, it must also have high-visibility while it is switching, and it must...

" 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

Chicago and North Western Railway

The Chicago and North Western Transportation Company was a Class I railroad in the Midwest United States. It was also known as the North Western. The railroad operated more than of track as of the turn of the 20th century, and over of track in seven states before retrenchment in the late 1970s...

 converted several E8B

EMD E8

The EMD E8 was a , A1A-A1A passenger train locomotive manufactured by General Motors' Electro-Motive Division of La Grange, Illinois. The cab version, or E8A, was manufactured from August, 1949 to December, 1953, and 449 were produced – 446 for U.S., and 3 for Canada...

 units purchased from the Union Pacific Railroad

Union Pacific Railroad

The Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....

. The cabs on the rebuilt units were referred to as "Crandall Cabs." The BNSF also experimented with a single GP60B to make it a "A unit" by using a 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.