Slug (railroad)
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In railroading, a slug is a version of a
diesel-electric locomotive A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), prime mover is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conv ...
which lacks a
prime mover Prime mover may refer to: Philosophy *Unmoved mover, a concept in Aristotle's writings Engineering * Prime mover (engine), motor, a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc) into energy ...
, and often a cab. It derives the electrical power needed to operate its
traction motor A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric vehicle, electric or hydrogen vehicles, elevators or electric multiple unit. Traction motors are used in electrically powered rail vehicles (ele ...
s and motor controls from a fully-powered mother locomotive. When coupled together it takes advantage of the excess current that the mother's diesel-electric locomotive produces at low speed, providing additional horsepower and braking at such operation without the expense of a full locomotive. A slug is distinct from a B–unit, which has both a prime mover and traction motors but no cab. A slug may retain an operator's cab to allow engineers to operate a train with the slug in the lead, or may have the cab and much of the body removed to save space and allow the operator in the mother better rear visibility.


Basic principles

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 The yard (symbol: yd) is an English unit of length in both the British imperial and US customary systems of measurement equalling 3 feet or 36 inches. Since 1959 it has been by international agreement standardized as exactly 0.914 ...
. At low speeds, a
diesel-electric locomotive A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), prime mover is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conv ...
prime mover is capable of producing more electricity than its traction motors can use effectively. Extra power would 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 force. In addition the load on each traction motor is reduced, which helps prevent overheating from excess current. Lacking a prime mover, slugs typically carry ballast to increase their weight and improve traction, often in the form of large blocks of
concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
. 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 diesel prime movers but having good traction motors.


Types

There are several types of slug, distinguished by intended use. This division is not absolute, and characteristics of one type may appear on another.


Yard slugs

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 A classification yard ( American and Canadian English ( Canadian National Railway use)), marshalling yard (British, Hong Kong, Indian, Australian, and Canadian English ( Canadian Pacific Railway use)) or shunting yard (Central Europe) is a railwa ...
switching, and transfer runs between yards. Some are radio controlled without an engineer in the cab.


Hump slugs

A hump slug is designed for even slower operation than a yard slug. They are often six-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 , while yard slugs would normally operate at up to .


Road slugs

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 brake Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. It is termed " rheostatic" if the generated electrical power is dissipated as heat in brake grid ...
s, 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. Road slugs may take several forms. A group of GP30, GP35,
GP38 The EMD GP38 is a four-axle diesel-electric locomotive built by General Motors Electro-Motive Division between January 1966 and December 1971. The locomotive's prime mover was an EMD 645 16-cylinder engine that generated . The company built 70 ...
, GP38AC, and GP40 locomotives were converted by
CSX CSX Transportation , known colloquially as simply CSX, is a Class I freight railroad operating in the Eastern United States and the Canadian provinces of Ontario and Quebec. The railroad operates approximately 21,000 route miles () of track. ...
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
radiator Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...
s and the removal of most of the access doors on the side of the body. 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 The Southern Pacific (or Espee from the railroad initials- SP) was an American Class I railroad network that existed from 1865 to 1996 and operated largely in the Western United States. The system was operated by various companies under the ...
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. 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, as the lack of a diesel prime mover provides them with a quiet and vibration-free ride.


Motors for added tractive effort

Motors for added tractive effort, or MATEs, 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.
Seaboard Coast Line Railroad The Seaboard Coast Line Railroad was a Class I railroad company operating in the Southeastern United States beginning in 1967. Its passenger operations were taken over by Amtrak in 1971. Eventually, the railroad was merged with its affiliate lin ...
's
GE U36B The GE U36B was a four-axle B-B diesel-electric locomotive produced by General Electric from 1969 to 1974. It was primarily used by the Seaboard Coast Line Railroad and its successors, although thirteen provided the power for the original ''Auto ...
fleet is the prime example: they were delivered new with special electrical cabinets to handle the two-to-four extra motors in a MATE. With a double-ended MATE, two U36Bs were equivalent to two U36Cs in every way. 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 The EMD SD40-2 is a C-C diesel-electric locomotive built by EMD from 1972 to 1989. The SD40-2 was introduced in January 1972 as part of EMD's '' Dash 2'' series, competing against the GE U30C and the ALCO Century 630. Although higher-horsepowe ...
s.


Terminology

Slugs are known by other names as well. Some are: *Drone (used by the Santa Fe) *MATE (motors for added tractive effort, used by GE) *RDMT (road MATE), used by
CSX CSX Transportation , known colloquially as simply CSX, is a Class I freight railroad operating in the Eastern United States and the Canadian provinces of Ontario and Quebec. The railroad operates approximately 21,000 route miles () of track. ...
*RDMATE (used by EMD) *TEBU (tractive effort booster unit, used by Morrison-Knudsen and
Southern Pacific The Southern Pacific (or Espee from the railroad initials- SP) was an American Class I railroad network that existed from 1865 to 1996 and operated largely in the Western United States. The system was operated by various companies under the ...
) *TEBC (tractive effort booster cab, BN/
BNSF BNSF Railway is one of the largest freight railroads in North America. One of seven North American Class I railroads, BNSF has 35,000 employees, of track in 28 states, and nearly 8,000 locomotives. It has three transcontinental routes that ...
cabbed slugs) *TEBCU (tractive effort booster cab unit, BN/
BNSF BNSF Railway is one of the largest freight railroads in North America. One of seven North American Class I railroads, BNSF has 35,000 employees, of track in 28 states, and nearly 8,000 locomotives. It has three transcontinental routes that ...
cabbed slugs) *Hump booster, used by
Canadian National The Canadian National Railway Company (french: Compagnie des chemins de fer nationaux du Canada) is a Canadian Class I freight railway headquartered in Montreal, Quebec, which serves Canada and the Midwestern and Southern United States. CN ...
The CCRCLs (control car remote control locomotives) used by
Union Pacific The Union Pacific Railroad , legally Union Pacific Railroad Company and often called simply Union Pacific, is a freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Paci ...
are sometimes called slugs. This designation is incorrect as the CCRCLs do not have traction motors.


Snails

A snail, often confused with a slug, is a cabless locomotive with a prime mover. However, instead of getting electricity from a separate unit, snails have no traction motors and therefore are incapable of operating under their own power. Like slugs, snails are rebuilt from damaged or worn out locomotives and retain the frame and bogies from the original unit. Snails are used for powering engineless units and have no cab or means to control themselves manually, except from a separate unit. SP rebuilt 9 F7Bs into snails for their
rotary snowplow A rotary snowplow (American English) or rotary snowplough is a piece of railroad snow removal equipment with a large circular set of blades on its front end that rotate to cut through the snow on the track ahead of it. The precursor to the rotary ...
s. Some of these are still in use today with
Union Pacific The Union Pacific Railroad , legally Union Pacific Railroad Company and often called simply Union Pacific, is a freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Paci ...
, along with their snowplow parent units, fighting snow on
Donner Pass Donner Pass is a mountain pass in the northern Sierra Nevada, above Donner Lake and Donner Memorial State Park about west of Truckee, California. Like the Sierra Nevada themselves, the pass has a steep approach from the east and a gradual appro ...
.


See also

*
Cow-calf In rail transport, a cow-calf (also cow and calf) locomotive is a set of switcher-type diesel locomotives. The set usually is a pair; some 3-unit sets (with two calves, also known as herds) were built, but this was rare. A cow is equipped with a ...
* Brake tender


References


External links

{{commons category, Slugs (railroad)
'Slugs' for extra tractive effort
-
Trains Magazine ''Trains'' is a monthly magazine about trains and railroads aimed at railroad enthusiasts and railroad industry employees. The magazine primarily covers railroad happenings in the United States and Canada, but has some articles on railroading els ...

Difference between a cabless booster, a slug, and a calf
- Trains Magazine Diesel locomotives