slug
An accessory to a diesel-electric locomotive, used to increase adhesive weight and allow full power to be applied at a lower speed. It has trucks with traction motors, but lacks a prime mover, being powered by electricity from the mother locomotive, and may or may not have a control cab.
slug: a powered trailer that feeds a locomotive's tractive effort
A slug is a coupled unit deployed behind a diesel-electric locomotive to multiply available adhesive weight without adding a second engine. It carries traction motors driven by electrical power drawn from the mother locomotive's generator, allowing the consist to apply full tractive effort at speeds where wheel slip would otherwise prevent grip. The slug has no prime mover of its own; it is entirely parasitic on the lead unit's power plant.
Slugs are built on standard freight-car trucks fitted with wound-rotor or AC induction motors mechanically coupled to the axles. Power flows through heavy electrical cables from the lead locomotive's main generator to the slug's motor contactors. A slug typically weighs 80 to 120 tons and can be equipped with either a full control cab (cabbed slug) or left as a headless unit (booster). Some slugs have regenerative capability, allowing them to feed power back into the electrical circuit during dynamic braking, though this feature is less common.
The slug's real value appears in low-speed, high-tractive-effort work: moving heavy consists from a standstill, pushing tonnage up sustained grades, or manoeuvring loaded cars in rail yards. At speeds below roughly 12 mph, wheel slip becomes the limiting constraint, not horsepower. A single lead unit powering a slug can exert more usable force than a solo locomotive of much greater engine displacement because the additional adhesive weight lets both the lead and slug wheels grip effectively.
Operational constraints and decline
Slugs demand compatible electrical architecture; an AC-traction lead locomotive cannot easily power an older DC-motored slug without converter equipment. As fleet electrification migrated to AC and as fuel efficiency became paramount, carriers built fewer new slugs. The need for physical cable connections and the cost of maintaining a second unit also worked against slug deployment. Many surviving slugs are regional workhorses in yards, mills, and short-haul operations rather than main-line revenue service.
The term 'slug' likely derives from its inert, low-profile form and slow-speed operation, though industrial etymology is rarely documented. In modern practice, the slug remains an economical workaround where a single larger prime mover would be uneconomical or impractical; it is most visible in heavy industrial switching and in railroads operating older diesel-electric rosters where compatible electrical plant justifies the maintenance burden.