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 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.
The first railway locomotives (19th century) were powered by steam engines. Because of this, the 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 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 nations 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 railfans. Steam locomotives remained in commercial use in parts of Mexico in to the late 1970s. Steam locomotives remain in regular use in China, where coal is a much more abundant resource than petroleum for diesel fuel. In some mountainous and high altitude rail lines, steam remains in use because it is less affected by reduced air pressure than is diesel.
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 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.
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; 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.
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 is fixed, all of which 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), 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.
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 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).
Steam
Diesel-mechanical
Diesel-electric
Diesel-hydraulic
Gas Turbine
Electric
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.