Slug

A slug — confusingly named, but the term is firmly established in North American railroad usage — is a heavy unmanned locomotive shell that carries powered traction motors on its bogies but has no diesel engine of its own. Instead, it draws electrical current from the generator of a paired "mother" locomotive coupled to it, putting that mother's surplus generator capacity to work at low speeds where the mother can't use it for itself. The combination — typically a four-axle mother paired with a four-axle slug — produces eight powered axles of tractive effort while consuming only the mother's fuel.

The point of a slug is the physics of starting and switching. At low speeds, a diesel-electric locomotive can generate far more electrical power than its own traction motors can absorb without overheating. The wasted capacity is dumped through resistance grids on the roof. Coupling a slug means those amps go into a second set of motors instead — doubling tractive effort at switching speed without doubling fuel consumption. Above maybe 25 mph the slug is dead weight, but at switching and yard speeds it's roughly equivalent to a second locomotive at half the operating cost.

Slugs are common in North American hump yards and heavy industrial switching operations. Visually they're distinguishable by their lack of exhaust stacks, their squat ballasted form (extra weight is added to maximise traction), and the cables running between them and their mother. Some railroads (CSX, Norfolk Southern, Conrail historically) built large slug fleets from cut-down older locomotives in the 1970s-80s; the practice continues today on a more modest scale.

The "slug" name is folk etymology, supposedly suggesting that the unit is heavy, slow, and dependent on a "mother" — apt enough, though the underlying engineering is more sophisticated than the joke suggests.