Cab unit
Encyclopedia
A cab unit and a carbody unit are body styles of 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...

s in railroad terminology. While closely related, they are not exactly the same.

With both body styles, a bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

-truss
Truss
In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in...

 design
Design
Design as a noun informally refers to a plan or convention for the construction of an object or a system while “to design” refers to making this plan...

 framework is used to make the body a structural element of the locomotive. The body rises above the locomotive frame, and extends the full width of the locomotive and along its length. The service walkways are inside the body.

This configuration gives a cab or carbody unit poor rear visibility compared with a 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...

. For that reason, cab or carbody units are mostly used in situations where rear visibility is not important, such as power for through freight and passenger trains. Cab and carbody units are also more aerodynamic than hood units, and pulled many of the streamliner
Streamliner
A streamliner is a vehicle incorporating streamlining in a shape providing reduced air resistance. The term is applied to high-speed railway trainsets of the 1930s to 1950s, and to their successor "bullet trains". Less commonly, the term is applied to fully faired recumbent bicycles...

 trains.

A and B unit

A cab unit is a carbody unit 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. Thus, a cab unit is also always an A unit
A unit
An A unit, in railroad terminology, is a 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...

 (a locomotive with a cab). By contrast, a carbody unit can be either an A unit, or a 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...

 (a locomotive without a cab).

Cowl unit

In recent years, locomotive manufacturers have switched from cab units to cowl unit
Cowl unit
A cowl unit is a body style of diesel locomotive. The terminology is a North American one, though similar locomotives exist elsewhere. A cowl unit is one with full-width enclosing bodywork, similar to the cab unit style of earlier locomotives, but unlike the cab unit style, the bodywork is merely...

s when a full-width body is desired. Since all the structural support on a cowl unit is in the frame of the locomotive, rather than the body, manufacturers can easily create full-width locomotives from hood unit designs by building cowl units.

Great Britain

Some British diesel locomotives, e.g. the British Rail Class 37
British Rail Class 37
The British Rail Class 37 is a diesel-electric locomotive. Also known as the English Electric Type 3, the Class was ordered as part of the British Rail modernisation plan....

, are cab units but the term "cab unit" is not used in Britain. The Class 37, like most British diesel and electric locomotives, has a cab at each end.
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