Locomotives

A locomotive is a rail vehicle that provides the motive power for pulling or pushing trains. Locomotives are differentiated from other self-propelled rail vehicles, such as multiple units, by the fact that a dedicated power unit pulls or pushes a train consist using traction motors located on the locomotive itself. The most common locomotive types today are diesel-electric and electric locomotives, with steam locomotives largely retired from regular service in most regions.

Steam locomotives were the first to achieve practical rail propulsion in the early 19th century. Improvements in steam engines and boiler design led to rapid adoption in Europe and North America, culminating in a period of expanding railway networks. In the 20th century, diesel-electric and electric locomotives replaced much of the steam fleet. Diesel-electric locomotives use a diesel engine to drive an electric generator that powers traction motors on the wheels; electric locomotives draw power from overhead lines or third-rail systems. Electric locomotives are common on busy passenger corridors and freight routes where electrification exists.

A locomotive comprises a prime mover, a propulsion transmission, wheelsets mounted on bogies, traction motors (in electric or diesel-electric designs), brakes, and control equipment. Wheel arrangements are described differently: steam locomotives are often classified by Whyte notation, while electric and diesel-electric units are categorized by other standards such as UIC or AAR. Performance is stated as power rating (horsepower or kilowatts) and tractive effort, and is influenced by adhesion, locomotive weight, and track conditions.

Today, locomotives power most mainline freight and passenger services in regions with non-electrified routes using diesel traction, while electrified networks rely on electric locomotives for high efficiency and speed. Heritage and preserved locomotives remain important for museums, tourism, and education.