locomotive (thing)

Conventionally, on a railroad, trains consist of a powered unit that pulls a number of unpowered railroad cars. More recently, on some trains, especially on suburban passenger routes, the trains are self-powered; all or many of the cars have their own engines or motors aboard (these are known as multiple units). Sometimes more than one locomotive is present, in a double-header, triple-header or, with modern locomotive, a consist of locomotives MU'ed together.

There are many reasons for this.

• Efficiency: it's more efficient to have one large power units than multiple smaller ones.
• Maintenance: a powered unit is more likely to fail than a non-powered one. Concentrating the power in a single locomotive means only the locomotive needs to be replaced if it fails.
• Safety: Historically, power was a dangerous thing. Until the mid 20th century, most trains were powered by steam locomotives, which, if not operated carefully, can prove quite dangerous. Modern diesel locomotives and electric locomotives are not quite so demanding of care.

Railroad locomotives of the 20th century are large, powerful beasts. A number of the largest steam locomotives built in the US weighed over a million pounds; horsepower at the drawbar attained over 8,000.

Locomotives can be divided into several categories by type of power. These include

• Steam locomotive: Power is generated by heating water, generally by burning fuel, in a sealed vessel called a boiler to produce high-pressure steam which is then converted into mechanical energy using pistons or a turbine. Steam locomotives can be found in a bewildering variety of types; in the US, they are generally described using the Whyte Notation.
• Diesel locomotive: Powered by a diesel engine, these have been the most common locomotives since the 1950s in the USA. There are three main subtypes, for the three major used methods of transferring the engine power to the wheels:
  • The diesel-electric is by far the most common and successful. The diesel engine drives a generator or alternator, driving electric motors that are mechanically connected to the wheels.
  • The diesel-hydraulic was relatively successful, but nowhere near to the success of the diesel-electric. The diesel engine drives a hydraulic transmission. Power is then transmitted to each truck via cardan shafts to gearing in each truck that in turn drives the wheels.
  • Diesel-mechanical locomotives are very similar to a car or truck; the engine is connected to the wheels via a geared transmission, which may be manually shifted or automatic.
• Electric locomotive: takes its power from an external electrical source, which it picks up via a pantograph or trolley pole contacting an overhead wire, or alternatively via a third rail shoe contacting an electrified third rail.