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A locomotive is a vehicle that provides the motive power for a
railway train. Traditionally, the locomotive
or locomotives are positioned at the front of a train, pulling passenger carriages and/or freight vehicles. This requires the locomotive to be moved from one end of the train to another when a change of
direction is required. However, it is now common for the locomotive for a passenger train to remain at the same end of the train.
A driving cab is provided in the outermost carriage, with controls which communicate with the locomotive through wiring along the
train. The train is thus pulled by the locomotive when moving in one direction, and pushed in the other. A variation of this
occurs when a train consists of a set of carriages with a locomotive at each end, both of which are controlled by a driver in the
leading locomotive.
Drivers are instructed by signals when they may start or must stop.
See railway signalling
Long freight trains sometimes have locomotives at the front and rear, and even in the middle of the train. This keeps the
train from stalling, and reduces the force on the couplings between the freight vehicles. In this case, control signals are
usually sent from the leading locomotive by radio.
Trains which do not have locomotives are referred to as multiple
units.
Steam
The first railway locomotives (19th century) were powered by steam engines. Because of this, some people took to informally calling the steam
locomotives themselves "steam engines". The steam locomotive remained by far the most common type of locomotive until after
World War II.
The first ever steam locomotive was built by Richard
Trevithick, and first ran on February 21, 1804, although it would take some years before steam locomotive design became efficient and economically
practical.
The all-time speed record for steam trains is held by an A4
Pacific class locomotive of the LNER in England, Mallard which pulling six cars (plus a dynamometer car)
reached 203 km/h (126 mph) on a slight downhill gradient down Stoke Bank on July 3,
1938. Locomotives from other countries such as Germany and the United States attained
speeds very close to this, and this is generally believed to be close to the practicable upper limit for the direct-coupled steam
locomotive.
Before the middle of the 20th century, electric and diesel locomotives
began replacing steam locomotives. By the end of the 1960s, most countries had completely
replaced steam locomotives in commercial service. Other designs, such as locomotives powered by gas turbines, have been
experimented with, but seen little use.
Well before the end of the 20th century, almost the only steam power still
in regular use in the USA and Western European countries was on railroads specifically aimed at tourists and/or railroad enthusiasts, known as railfans. Steam locomotives remained in commercial use in parts of Mexico into the late 1970s. Steam locomotives remain in regular use in
India and China, where coal is a much more abundant resource than petroleum for diesel
fuel. In some mountainous and high altitude rail lines, steam engines remain in use because they are less affected by reduced air
pressure than diesel engines.
Diesel-mechanical
Diesel locomotives differ in the form of transmission used to convey the power from a diesel engine (or engines) to the wheels. The most simple form of transmission is by means of a gearbox, in the same way as on road vehicles. Diesel trains or locomotives which use this are
called diesel-mechanical.
It has however, been found impossible to build a gearbox which can cope with a power output of more than 400 horsepower (300 kW) without breaking, despite a number of attempts to do so. Therefore
this type of transmission is only suitable for low powered shunting locomotives, or lightweight multiple units or railcars.
For more powerful locomotives other types of transmission have to be used.
Diesel-electric
Twin diesel-electric locomotives of the Union Pacific refueling at Dunsmuir, California
The most common form of transmission is electric; a locomotive using electric transmission is known as a diesel-electric locomotive. With this system, the diesel engine drives a generator or alternator; the electrical power produced then drives the wheels using electric motors. In effect, such a locomotive is an electric locomotive which carries its own generating
station along with it.
Early diesel-electrics were switching engines used to move rail cars around in rail yards. The first went into service in
1924. A decade later, the technology first began to be applied to regular rail service as
streamliners went into service. Actually, a gasoline-electric system powered the first such train, but diesel-electric systems soon proved to be more
cost-effective because of higher efficiency and lower maintenance costs. The fuel for one early high-speed run from Chicago, Illinois to Denver, Colorado only cost US$14.64
(in 1934 dollars).
Diesel-hydraulic
Alternatively, diesel-hydraulic locomotives use hydraulic transmission to
convey the power from the diesel engine to the wheels. On this type of locomotive, the power is transmitted to the wheels by
means of a device called a torque converter. A torque
converter consists of three main parts, two of which rotate, and one which is fixed. All three main parts are sealed in a housing
filled with oil.
The inner rotating part of a torque converter is called a centrifugal pump (or impellar), the outer part is called a
turbine wheel (or driven wheel), and between them is a fixed guide wheel. All of these parts have specially shaped
blades to control the flow of oil.
The centrifugal pump is connected directly to the diesel engine, and the turbine wheel is connected to an axle which drives the wheels.
As the diesel engine rotates the centrifugal pump, oil is forced outwards at high pressure. The oil is forced through the
blades of the fixed guide wheel and then through the blades of the turbine wheel, which causes it to rotate and thus turn the
axle and the wheels. The oil is then pumped around the circuit again and again.
Diesel-hydraulic locomotives are slightly more efficient than diesel-electrics but are mechanically more complicated and more
likely to break down. They are now greatly outnumbered by diesel-electrics.
Gas Turbine
Locomotives powered by gas turbines, were developed in many countries in
the decades after World War II. These used jet-type engines (similar to the
turboshaft engines in a turbine helicopter) driving an output shaft. The normal
method of transmitting power to the wheels involved an electrical transmission similar to a diesel-electric locomotive -- the turbines running at constant speed driving a generator, feeding to
large electric motors driving the wheels.
Gas turbine locomotives are very powerful, but also very noisy (they sounded rather like a jet aircraft). Their efficiency was
quite low, but this was initially not a problem; fuel was cheap, and some gas turbines were fuelled with cheap 'Bunker C' heavy
oil. This cheap fuel source vanished when improved refinery techniques allowed it to be 'cracked' into lighter petroleum grades.
After the oil crisis in the 1970s and
the rise in fuel costs, gas turbine locomotives became uneconomic to run, and many were taken out of service. This type of
locomotive is now rare.
Electric
Electric locomotives are externally supplied with electric power, either through an overhead pickup or through a third rail. While the cost of
electrifying track is rather high, electric trains and locomotives are significantly cheaper to run than diesel ones, and are
capable of superior acceleration as well as regenerative braking, making them ideal for passenger service in
densely populated areas. Almost all high speed train systems (e.g.
ICE, TGV, bullet train) use electric locomotives, because the power needed for such performance is not easily carried on
board.
However in the 1970s British Rail
in the United Kingdom, developed a high-speed diesel electric locomotive called the High Speed Train, which is capable of reaching speeds of up to 284 km/h (176 mph), although in service it
only reaches speeds of 200 km/h (125 mph).
The world speed record for a wheeled train was set in 1990 by a French TGV which reached a speed of 515 km/h (320 mph).
While recently designed electrified railway systems invariably operate on alternating current, many existing direct
current systems are still in use—e.g. in South Africa, Spain, Switzerland and the United Kingdom, Netherlands (1500 V), Belgium (3000 V). See also: Railway electrification system.
Magnetic Levitation
The newest technology in locomotives is magnetic
levitation (maglev). These electrically powered trains have a special open motor which floats the train above the rail
without the need for wheels. This greatly reduces friction. Very few systems are in service and the cost is very high. The
experimental Japanese magnetic levitation train has reached 552 km/h.
See also
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