Slug (railroad)
A railroad slug is an accessory to a locomotive. A slugs has trucks with traction motors but is unable to move about under its own power as it does not contain a prime mover. Instead, it is connected to a locomotive, called the mother, which provides current to operate the traction motors. Sometimes a locomotive will be connected to each end of the slug and both will provide power to it. Slugs are used in situations where high tractive effort is more important than extra horsepower, such as switching operations in yards. At low speeds, a locomotive's prime mover produces more electric power than its traction motors can use. The extra current would simply cause the wheels to slip. A slug effectively adds axles to the locomotive, increasing both the pulling and braking power. Some slugs are equipped with dynamic brakes. These can be readily identified by looking for a noticeable hump on top of the hood. Slugs carry ballast to increase their weight and improve traction. Large blocks of concrete are frequently used, though some slugs carry fuel tanks and thus function as fuel tenders for the attached locomotives.
Slugs can be built new or converted from existing locomotives. In the latter case, the locomotive body is frequently cut down and the cab eliminated, as in the first image. One notable exception is the slug fleet operated by CSX. Those closely resemble 'real' locomotives. They have cabs, and sometimes operate at the front of the train. The only real signs that it is a slug is the nearly silent operation and the absence of radiator grilles. See the second image for an example.
Most slugs have four axles and are used in yards for moving long strings of cars at a walking pace. These are typically referred to as yard slugs. The combination of a locomotive and a yard slug is often called a cow-and-calf set. Some slugs have six axles and are called road slugs. These are used at higher speeds, usually up to 30 miles per hour. As the train accelerates, less power is available for the slug to use. Eventually the slug will be disconnected from the locomotive's power circuit. At that point, it becomes just another car in the train.
Slugs are known by other names as well, such as drone (used by the Santa Fe), MATE (Motors for Added Tractive Effort, used by GE), TEBU (Tractive Effort Booster Unit, used by Morrison Knudsen) or the RDMATE (used by EMD).
The CCRCLs (Control Car Remote Control Locomotives) used by Union Pacific are sometimes called slugs. This designation is incorrect as the CCRCLs do not have traction motors.
The slug concept is similar to switching from two wheel drive to four wheel drive in an automobile.