Under the Whyte notation for the classification of steam locomotives, 2-4-2 represents the wheel arrangement of two leading wheels on one axle, four powered and coupled driving wheels on two axles, and two trailing wheels on one axle. The type is sometimes named Columbia after an early locomotive of the 2-4-2 arrangement.
Other equivalent classifications are:
- UIC classification: 1B1 (also known as German and Italian classifications)
- French classification: 121
- Turkish classification: 24
- Swiss classification: 2/4
When the leading and trailing wheels are in swivelling trucks, the equivalent UIC classification is 1'B1'.
The wheel arrangement was widely used on passenger tank locomotives during the last three decades of the nineteenth and the first decade of the twentieth centuries. The vast majority of 2-4-2 locomotives were tank locomotives, designated 2-4-2T. The symmetrical wheel arrangement suits a tank locomotive that is used to work in either direction.
While a number of 2-4-2 tender locomotives were built, larger tender locomotive types soon became dominant.
In 1877, when the New Zealand Railways needed new motive power, the road turned to the Rogers Locomotive Works who supplied eight 2-4-2 tender locomotives between 1877 and 1879 that were designated the "K" class. These were the first American-built locomotives in New Zealand and proved to be quite successful.
Three of these locomotives have been preserved. No. K88 Washington was used on the first through train between Christchurch and Dunedin in 1877. After fifty years of service, Washington was dumped in the Oreti River, Southland, as a flood protection measure. In 1974 the locomotive was exhumed from her watery grave and, over the next eight years, restored to full active service.
Sister locomotives numbers K92 and K94 have also been salvaged from the Oreti river. No. K92 has been restored to full active service and has re-established her position on the Kingston Flyer train, which was made famous by the K class at the end of the 19th century.
The earliest British use of the 2-4-2 wheel arrangement appears to have been no. 21 White Raven, supplied to the St Helens Railway by James Cross of Sutton Works in 1863. It was soon rebuilt as a 2-4-0 tender locomotive and eventually passed into the stock of the London and North Western Railway (L&NWR).
Other railway companies that built large numbers of the type included the Lancashire and Yorkshire Railway (L&YR) with 330 locomotives built between 1889 and 1911, the North Eastern Railway with 60 locomotives built between 1886 and 1892 and the Manchester, Sheffield and Lincolnshire Railway with 49 locomotives built between 1889 and 1898. One of John Aspinall's Class 5 locomotives, built for the L&YR in 1889, is preserved at the National Railway Museum, York.
- The character Sammy from Sammy the Shunter is based on the United Kingdom design of the 2-4-2 engine.
- Two characters in the 1991 film The Little Engine That Could have this type of wheel arrangement.
- Albert from The Railway Series has this wheel arrangement.