Slug (railroad)

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.

Lacking a prime mover, a slug typically carries ballast to improve traction, often in the form of large blocks of concrete.

Conversion has enjoyed popularity as a way to reuse otherwise obsolete locomotives, especially those with worn-out diesel prime movers but working traction motors.

It usually retains dynamic braking, a feature useless at the low speeds encountered in switching service, and it may be equipped to serve as a fuel tender for an attached mother locomotive.

As speed increases it is disconnected from the power circuit and can function as a control cab if in the lead, or simply as an unpowered car in the consist.

Externally they retain the general appearance of powered diesel–electric locomotives, though they can be identified by the absence of radiators and most of the access doors on the sides.

When at one end of a set, train crews will often go to great lengths to arrange for the slug to be the leading unit, providing them with a more pleasant experience without the added noise and vibration from a diesel engine.

Seaboard Coast Line Railroad's GE U36B fleet is the prime example: they were delivered new with special electrical cabinets to handle the two-to-four extra motors in a MATE.

Some railroads emulated this concept by using two GP40s, then swapping in six-axle Dash 2 electrical cabinets, and then connecting the GP40s to a double-ended slug rebuilt from an old locomotive.