A locomotive chassis heavily ballasted with concrete or steel and equipped with traction motors, but lacking a diesel engine. It is permanently coupled to a normal locomotive, known as a mother, drawing excess electrical power from the mother's main generator at low speeds. Slugs maximize tractive effort and adhesion for heavy drag freight and yard switching without the fuel cost of a second engine.
We've a pair of GP40s and a yard slug to pull that heavy cut of coal cars up the hump.
Slugs are highly effective at speeds under 15 mph, after which the mother locomotive needs all its own electricity to maintain speed.
Related terms
Mother
A fully equipped diesel-electric locomotive that provides electrical power to a semi-permanently coupled slug.
Yard slug
A low-profile, unpowered locomotive chassis equipped with traction motors to provide extra pulling force in a switching yard.
Road slug
A specialized locomotive unit without a prime mover used to provide extra traction at higher speeds.
Hump slug
A heavy, motor-equipped locomotive unit without a diesel engine used to provide extra traction at very low speeds.