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. When coupled together it takes advantage of the excess current that the mother's diesel-electric locomotive produces at low speed, providing additional horsepower and braking at such operation 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 retain an operator's cab to allow engineers to operate a train with the slug in the lead, or may have the cab and much of the body removed to save space and allow the operator in the mother better rear visibility.