With both body styles, a bridge-truss design 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. 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 trains.
A and B unit
A cab unit is a carbody unit with a driving cab, or crew compartment. Thus, a cab unit is also always an A unit (a locomotive with a cab). By contrast, a carbody unit can be either an A unit, or a B unit (a locomotive without a cab).
In recent years, locomotive manufacturers have switched from cab units to cowl units 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.
Most British diesel locomotives, e.g. the British Rail Class 37, 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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