locomotive, vehicle used to pull a train of unpowered railroad cars.
Types of Locomotives
The steam-powered locomotive played a key role during the development and golden age of railroading, but, despite its long and picturesque history, it has been superseded in developed nations by electric and diesel-electric locomotives for economic and environmental reasons. The few steam locomotives that remain in operation in developed nations are mostly nostalgic relics used chiefly to pull tourist trains.
The reciprocating steam locomotive is a self-contained power unit consisting essentially of a steam engine and a boiler with fuel and water supplies. Superheated steam, controlled by a throttle, is admitted to the cylinders by a suitable valve arrangement, the pressure on the pistons being transmitted through the main rod to the driving wheels. The driving wheels, which vary in number, are connected by side rods. Steam locomotives are usually classified under the Whyte system, that is, by the number and arrangement of the wheels; for example, an engine classified as 2–6–0 has one pair of wheels under the front truck, three pairs of coupled or driving wheels, and no wheels under the trailing truck. In some cases the truck wheels of the tender (fuel carrier) are added.
Electric locomotives range from the small type used in factories and coal mines for local hauling to the large engines used on railroads. Electric locomotives generally have two or more motors. Power is collected from an electric trolley, or pantograph, running on an overhead wire or from a third rail at one side of the track. Battery locomotives, used only for local haulage, carry electric storage batteries that act as their primary source of power. Electric railroad locomotives are used chiefly on steep grades and on runs of high traffic density; although highly efficient they are not more widely used because of the cost of electric substations and overhead wires or third rails.
Diesel-electric locomotives were introduced in the United States in 1924, and have become the most widely used type of locomotive. The modern diesel-electric locomotive is a self-contained, electrically propelled unit. Like the electric locomotive, it has electric drive, in the form of traction motors driving the axles and controlled with electronic controls. It also has many of the same auxiliary systems for cooling, lighting, heating, and braking. It differs principally in that it has its own generating station instead of being connected to a remote generating station through overhead wires or a third rail. The generating station consists of a large diesel engine coupled to an alternator or generator that provides the power for the traction motors. These motors drive the driving wheels by means of spur gears. The ratio of the gearing regulates the hauling power and maximum speed of the locomotive. A modern diesel-electric locomotive produces about 35% of the power of a electric locomotive of similar weight. Diesel-mechanical locomotives have a direct mechanical link consisting of a clutch and a series of gears and shafts between the engine and the wheels, similar to the transmission in an automobile. Because mechanical drives deliver less power to the wheels than electric and diesel-electric systems, they are only used with the smallest locomotives. In diesel-hydraulic locomotives the engine drives a torque converter, which uses fluids under pressure to transmit and regulate power to the wheels. Hydraulic drives are little used in the United States but are widely used in some countries, such as Germany.
Gas turbine–electric locomotives are similar to the diesel-electric but use a gas turbine to drive the generator. The technology is used primarily on turbotrains, high-speed passenger trains that do not have locomotives but instead are powered by units built into one or more of their cars.
Development of the Locomotive
Richard Trevithick, a British engineer and inventor, built and operated (1803–4) the first successful steam engine locomotive for hauling cars on a track. The British engineer George Stephenson built his first locomotive, the Blucher, in 1814, and in 1829 he demonstrated the practicability of the steam engine for commercial transportation; his locomotive, the Rocket, attained 29 mi per hr (47 km per hr). The first American-built locomotive was designed and tested on a private track by the American engineer John Stevens in 1826. The English-built Stourbridge Lion, imported c.1829, was not a commercial success, being too heavy for American tracks.
The Tom Thumb (1830), built by Peter Cooper, an American manufacturer, for the Baltimore & Ohio RR, was the first practical American-built locomotive. The American manufacturer Matthias Baldwin's first locomotive, Old Ironsides, built in 1832, long remained in operation. In 1832 the American engineer John B. Jervis built the first locomotive with a swivel truck, a wheel assembly on which part of the body was mounted. Placed at the forward end of a locomotive, a swivel truck permitted a locomotive to negotiate curves more safely. In 1865, Robert F. Fairlie produced an articulated (jointed) locomotive that could traverse the sharp curves of passes through the western mountains. Electric locomotives were introduced on the Baltimore & Ohio RR in 1895, and diesel locomotives—introduced in yard service in 1924—were in general use by 1935.
See C. Garrat, The Last of Steam (1980); D. Weitzman, Superpower: The Making of a Steam Locomotive (1987); R. Loewy, Locomotive (1988); E. A. Haine, The Steam Locomotive (1990); B. Solomon, The American Steam Locomotive (1998); B. Solomon, The American Diesel Locomotive (2000); see also bibliography under steam engine.