
A B unit, in railroad terminology, is a locomotive unit (generally a diesel locomotive) which does not have a driving cab or crew compartment, and must therefore be controlled from another, coupled locomotive with a driving 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 reasons railroads ordered B units included the fact that a B unit was slightly cheaper. With no driving cab, B units lack windshields, crew seats, radios, heating, and air conditioning.