Slug (railroad)

A railroad slug is an accessory to a diesel-electric locomotive. It has trucks with traction motors but, unlike a B unit, is unable to move about under its own power, as it does not contain a prime mover to produce electricity, and there may or may not be a cab for an operator. Instead, it is connected to a locomotive, called the mother, which provides the needed electrical power to operate the traction motors, and the motor controls.

A slug is used to increase adhesive weight, allowing full power to be applied at a lower speed, thus allowing a higher maximum tractive effort. They are often used in low-speed operations such as switching operations in yards. At low speeds, a diesel-electric locomotive prime mover is capable of producing more electricity than its traction motors can use effectively. Extra power would simply cause the wheels to slip and possibly overheat the traction motors. A slug increases the number of traction motors available to the locomotive, increasing both the pulling and braking power. In addition the load on each traction motor is reduced, which helps prevent overheating from excess current. Slugs typically carry ballast to increase their weight and improve traction. Large blocks of concrete are frequently used for this purpose, substituting for the weight of the now-absent prime mover.

Slugs can be built new or converted from existing locomotives. Conversion has enjoyed popularity as a way to reuse otherwise obsolete locomotives, especially those with worn-out diesels and good traction motors.

There are several types of slug, distinguished by intended use. Note that as with diesel locomotives in general, this division is not absolute, and characteristics of one type may appear on another.

A yard slug is designed for switching, and therefore is built to increase visibility in low speed operation. It has a low body and no cab, allowing the engineer or driver in the powered unit to see past it. Mother-slug sets are used in heavy switching, hump yard switching, and transfer runs between yards. Some are radio controlled without an operating engineer present in the cab.

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