B–unit

A B unit, in railroad terminology, is a locomotive unit (generally a diesel locomotive) which does not have a control cab or crew compartment, and must therefore be operated in tandem with another coupled locomotive with a cab (an A unit). The terms booster unit and cabless are also used. The concept is largely confined to North America. Elsewhere, locomotives without driving cabs are rare.

The term primarily is applied to freight locomotives, but can be applied to passenger multiple units as well in some cases for when motor cars are put in the middle of trains. In practice however the term is rarely used and in many circumstances the non-cab cars are directly or indirectly permanently attached to a cab car.