Origins

A steam locomotive is a locomotive powered by steam. Steam locomotives dominated rail traction from the mid 19th century until the mid 20th century, after which they were superseded by diesel and electric locomotives.

Many consider the steam locomotive to be one of the most fascinating mechanical devices ever created, passing steam locomotives often grabbing the attention of bystanders.

The earliest railways employed horses to draw carts over the track. As steam engines were developed in the 1700s, various attempts were made to apply these to road and railroad use. The first attempts were made in Great Britain; the earliest steam locomotive was built in 1804 by Richard Trevithick and Andrew Vivian. It ran with mixed success on the narrow gauge "Penydarren" (Merthyr Tydfil) tramway in Wales. Then followed the successful twin cylinder locomotives built by Matthew Murray for the edge railed Middleton Railway in 1812. These early efforts culminated in 1829 with Stephenson's Rocket, which was the first viable mainline locomotive.

Trevithick's locomotive, 1804. Inspired by British success, the United States started developing steam locomotives in 1829 with the Baltimore and Ohio Railroad's Tom Thumb. This was the first locomotive to run in America, although it was intended as a demonstration of the potential of steam traction, rather than as a revenue-earning locomotive. The first successful steam railway in the US was the South Carolina Railroad whose inaugural train ran in December 1830 hauled by the Best Friend of Charleston. Many of the earliest locomotives for American railroads were imported from England, including the Stourbridge Lion and the John Bull, but a national locomotive manufacturing industry was quickly established, with locomotives like the DeWitt Clinton being built in the 1830s.

Categorisation

Steam locomotives are categorised by their wheel arrangement. The two dominant systems for this are the Whyte notation and UIC classification.

The Whyte notation, used in most English speaking and Commonwealth countries, represents each set of wheels with a number. Different arrangements were given names which usually reflect the first usage of the arrangement; for instance the "Santa Fe" type (2-10-2) is so called because the first examples were built for the Atchison, Topeka and Santa Fe Railroad. These names were informally given and varied according to region and even politics.

The UIC classification is used mostly in European countries apart from the United Kingdom. It designates consecutive pairs of wheels (informally "axles") with a number for non-driving wheels and a capital letter for driving wheels (A=1, B=2 etc). So a Whyte 4-6-2 designation would be an equivalent to a 2-C-1 UIC designation.

On many railroads, locomotives were organised into classes. These broadly represented locomotives which could be substituted for each other in service, but most commonly a class represented a single design. As a rule classes were assigned some sort of code, generally based on the wheel arrangement. Classes also commonly acquired nicknames representing notable (and sometimes uncomplimentary) features of the locomotives.