Slug (railroad)

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In railroading, a slug is a version of a diesel–electric locomotive which lacks a prime mover and often a cab. It derives the electrical power needed to operate its traction motors and motor controls from a fully-powered mother locomotive. At low speeds the drawing (or braking) force a diesel–electric locomotive can produce is often limited by its traction motors or the grip of its drive wheels on the track, not the capability of its diesel engine(s). A slug adds more traction motors and drive wheels to both use more of the power the mother's engine can produce that cannot otherwise be used at low speeds and provide better braking, without the expense of a full locomotive.^[1]

A slug is distinct from a B unit, which has both a prime mover and traction motors but no cab. A slug may have an operator's cab to allow engineers to operate a train with the slug in the lead, or the cab and much of the body can be omitted to reduce cost and size, the latter allowing better visibility for an operator in the mother.