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A slug or a "power unit" is an accessory to a diesel 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, engine, or motor to produce electricity. Instead, it is connected to a locomotive (otherwise known as the "mother") which provides current to operate the traction motors. They are often found on freight and low capacity passenger trains.
There are several types of slugs 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.
A hump slug is designed for even slower operation than a yard slug. They are often 6 axle slugs and are often paired with lower powered six axle locomotives. They are designed for the specialized purpose of pushing a long cut of cars over a hump at 2 to 3 miles per hour, while yard slugs would normally operate at up to 10 to 15 mph.
Road slugs are intended to serve as part of a regular locomotive consist for road haulage, and as a result have certain adaptations to suit them for this service. They usually retain dynamic brakes, a feature useless at the low speeds encountered in switching service, and they may be equipped to serve as fuel tenders for the attached "mother" locomotives.
In operation, they are used to provide extra traction at low speeds. As speed increases they are disconnected from the power circuit and function as a control cab if they are in the lead, or simply as an unpowered car in the consist. In braking they augment the powered locomotives, both during dynamic and air brake application. They retain the cab and its controls, and therefore multiple unit control allows them to function as the lead in a string of units. The TEBU units created on the Southern Pacific Railroad from General Electric U25Bs, on the other hand, were cabless; this potential operational deficiency was compensated for by putting them as the center unit of a set of three.
MATE's (or simply "mate's") appear similar to slugs, but their design is different. Instead of siphoning-off power as a slug does, the axles in a MATE are fully connected into the transition series in the locomotive it is connected to. A double-ended MATE (a MATE with connections on both ends) turns two four-axle locomotives into the equivalent of two fully fledged six-axle locomotives. A single-ended mate turns a four-axle locomotive into the equivalent of an eight-axle locomotive. MATEs do not cut out at speed as the motors are fully included in the series–parallel transition stages. SCL's GE U36B fleet is the prime example: they were delivered new with special electrical cabinets to handle the 2 to 4 extra motors in a MATE. With a double-ended MATE, two U36Bs were equivalent to two U36Cs in every way.
The term "slug" is actually a nickname which derives from railfans and employees who often think that power units often move like slugs, or "slug around".
A similar concept exists and is used in the UK, yet "slugs" are known as "slaves"; to whom the nickname derives from the power unit literally being the "partner" locomotive's "slave".
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. The result is the equivalent of two SD40-2's.
Road slugs may take several forms. Such as CCRCL (Control Car Remote Control Locomotive) units rebuilt from Union Pacific.
CSX's entire fleet of GP30 and GP35 locomotives were converted, and operated as half of "mother-slug" pairs. Externally they retain the general appearance of powered diesel-electric locomotives, though they can be identified by the lack of radiators and the removal of most of the access doors on the side of the body.
When at one end of a set, locomotive crews will often go to great lengths to arrange for the slug to be the leading unit, as the lack of a diesel prime mover provides them with a quiet and vibration-free ride.