A locomotive is a railway vehicle that provides the motive power for a train, and has no payload capacity of its own; its sole purpose is to provide power. Many trains feature self-propelled payload-carrying vehicles; these are not normally considered locomotives, and may be referred to as multiple units or railcars; the use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight (see German CargoSprinter ). Vehicles which provide the motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars .
Traditionally, locomotives haul their trains. Increasingly common these days in passenger service is push-pull operation, where the locomotives push the trains in one direction, controlled from a control cab at the opposite end of the train.
Benefits of locomotives
There are many reasons why the motive power for trains has been traditionally isolated in a locomotive, rather than self-propelled vehicles. These include:
- Ease of maintenance - it is easier to maintain one locomotive than many self-propelled cars.
- Safety - it is often safer to locate the train's power systems away from passengers. This was particularly the case for the steam locomotive, but still has some relevance.
- Easy replacement of motive power - should the locomotive break down, it is easy to replace it with a new one. Failure of the motive power unit does not require taking the whole train out of service.
- Efficiency - idle trains do not waste expensive motive power resources. Separate locomotives mean that the costly motive power assets can be moved around as needed.
- Obsolescence cycles - separating the motive power from the payload-hauling cars means that either can be replaced without affecting the other. At some times, locomotives have become obsolete when their cars are not, or vice versa.
Classification by motive power
Locomotives may generate mechanical work from fuel, or they may take power from an outside source. It is common to classify locomotives by their means of providing motive work: the common ones include:
A steam locomotive at the Gare du Nord
, Paris, France, in 1930
The first railway locomotives (19th century) were powered by steam engines, first by burning wood, later coal or oil. Because of the steam engine, 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 age of steam correlates highly to the coal era . Steam locomotives tremendously helped the first exploitation of coal mines, and their demise beginning in the 1960s heralded the end of steam locomotives.
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 Fairy Queen, built in 1855; plying between New Delhi and Alwar in India, is the longest running steam locomotive in regular service in the world, but the John Bull, built in 1831, is currently the oldest operable steam locomotive. John Bull is preserved in mostly static display at the Smithsonian Institution in Washington, DC.
The all-time speed record for steam trains is held by an LNER Class A4 4-6-2 Pacific locomotive of the LNER in England, number 4468 Mallard, which pulling six cars (plus a dynamometer car) reached 126 mph (203 km/h) on a slight downhill gradient down Stoke Bank on July 3, 1938. Aerodynamic passenger 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. To reach faster speeds and a larger action radius, tender locomotives replaced tank locomotives for long distance activities.
Before the middle of the 20th century, electric and diesel-electric locomotives began replacing steam locomotives. A very important factor for the end of the steam era is the depletion and consequent closing of coal mines (especially in Europe). By the end of the 1960s-1970s, most western countries had completely replaced steam locomotives in commercial service. Freight locomotives generally were replaced later. Other designs, such as locomotives powered by gas turbines, have been experimented with, but seen little use.
By the end of the 20th century, almost the only steam power still in regular use in North America and Western European countries was on railroads specifically aimed at tourists and/or railroad enthusiasts, known as railfans or train spotters. Steam locomotives remained in commercial use in parts of Mexico into the late 1970s. Steam locomotives are in regular use in China, where coal is a much more abundant resource than petroleum for diesel fuel. India has switched in last decade from steam-powered trains to electric- and diesel-powered trains. 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 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 impractical 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.
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).
In the 1970s, British Rail in the United Kingdom developed a high-speed diesel-electric train called the High Speed Train or HST. This train consists of two Class 43 locomotives (also known as power cars), one at each end, and a number of "Mark 3" carriages (usually 8). A complete HST set was originally designated as a Class 253 or 254 diesel multiple unit (DMU), but due to the frequent exchanges between sets the power cars were reclassified as locomotives and given class number 43. The unpowered carriages were simultaneously reclassifed as individual coaches - the number of a DMU set should identify all its associated carriages as well.
The prototype HST (designated Class 252) holds the world speed record for diesel traction, having reached a speed of 143 mph, although the operating speed of the production HST in service is 125 mph (200km/h), hence the name "Inter-City 125".
A variant of the Intercity 125 (the "XPT ") is in service on Australian railways, but with a lower top speed and different carriages.
Dual-mode diesel-electric/third rail locomotives are operated by the Long Island Rail Road between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in city tunnels. For the same reasons, Amtrak operates a fleet of dual-mode locomotives on some lines in the northeast of the United States.
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. Many diesel-hydraulic multiple units also have a "fluid flywheel" which acts as a "second gear" for running at higher speeds.
The inner rotating part of a torque converter is called a centrifugal pump (or impeller), 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. In the US and Canada, they are now greatly outnumbered by diesel-electric locomotives, while they remain dominant in Europe.
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.
Main article: Electric locomotive
The electric locomotive is 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.
The world speed record for a wheeled train was set in 1990 by a French TGV which reached a speed of 515.3 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, and the United Kingdom; Netherlands (1500 V); Belgium, Italy, Poland (3000 V), and the cites of Mumbai and Chicago (who will be switched to AC by 2025).
See also: Railway electrification system
Transrapid maglev train on the test track at Emsland
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.
The transrapid maglev train connects Shanghai's airport with the city.
Classification by use
The three main categories of locomotives are often subdivided in their usage in rail transport operations. There are passenger locomotives, freight locomotives and switcher (or shunter) locomotives. These categories mainly depend on manoeuvrability, traction power and speed. Some locomotives are designed to work in mountain railways.