A cab unit and a carbody unit are body styles of locomotives in North American railroad terminology. While closely related, they are not exactly the same. A carbody unit may be either a cabless booster unit controlled from a linked cab unit, or a cab unit that contains its own controls.
With both body styles, a bridge-truss design framework is used to make the body a structural element of the locomotive. The body extends the full width and length of the locomotive. The service walkways are inside the body.
Carbody units, gaining rigidity from the body trusswork, require less structural weight to achieve rigidity than do locomotives with non-structural bodies. For that reason, carbody construction was favored to increase the power-to-weight ratio for early diesel locomotives, before the power available with diesel technology was increased. Recent years have seen carbody construction revived in the quest for greater fuel efficiency with passenger locomotives.
The full-width body gives a carbody cab 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).
Passenger-oriented cab units
A cowl unit is an adaptation of the hood unit design with a full-width body. Since all the structural support on a cowl unit is in the frame of the locomotive, rather than the body as with a cab unit, manufacturers can easily create full-width locomotives from hood unit designs by building cowl units. Cowl units were introduced in passenger service for style reasons during the early 1970s.
Cab units were not generally used in Great Britain. The traditional makers continued to use heavyweight frames and cowl units instead.
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.