Copied and pasted from Railroad terminology.
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.
Cow-calf. ... Cow and calf. In North American railroading, a cow-calf (also cow and calf) locomotive is a set of switcher-type diesel locomotives. The set usually is a pair; some 3-unit sets (with two calves) were built, but this was rare. A cow is equipped with a driving cab; a calf is not.
A railroad slug is an accessory to a diesel-electric locomotive. It has trucks with traction motors but, unlike a B unit, is unable to move about under its own power, as it does not contain a prime mover to produce electricity, and there may or may not be a cab for an operator.