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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 2-4-2 arrangement.
Other equivalent classifications are:
- UIC classification: 1B1 (also known as German classification and Italian classification)
- French classification: 121
- Turkish classification: 24
- Swiss classification: 2/4
The equivalent UIC classification is 1'B1', if the leading and trailing wheels are in swivelling trucks.
While a number of 2-4-2 tender locomotives were built, larger types soon became dominant. The vast majority of 2-4-2s have been tank locomotives, designated 2-4-2T. The symmetrical arrangement suits a tank locomotive, which may travel in either direction.
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.
In 1877, when the New Zealand Railway needed new motive power, the road turned to the Rogers Locomotive Works, who supplied eight 2-4-2 tender engines that were designated as the "K" class. These were the first American-built engines in New Zealand, and had been quite successful.
The earliest UK use of the 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 afterwards rebuilt as a 2-4-0 tender locomotive and passed into the stock of the London and North Western Railway (L&NWR).
In 1864, Robert Sinclair of the Great Eastern Railway designed the first of six 2-4-2T tank classes built by the railway, eventually totalling 262 locomotives by 1912 (see Locomotives of the Great Eastern Railway). Francis Webb of the LN&WR also designed two classes, which eventually totalled 380 locomotives, built between 1879 and 1898 (see Locomotives of the London and North Western Railway). Other companies to build large numbers of the type included the Lancashire and Yorkshire Railway (L&YR) (330 locomotive 1889-1911), the North Eastern Railway (60 locomotives 1886-1892), and the Manchester, Sheffield and Lincolnshire Railway (49 locomotive 1889-1898).
Three New Zealand Railways K class tender locomotives, built between 1877 and 1879 by the Rogers Locomotive Works, have been preserved. K88 "Washington" was used on the first through train between Christchurch and Dunedin in 1877. After 50 years of service, "Washington" was dumped in the Oreti River, Southland, as a flood protection measure. In 1974, "Washington" was exhumed from her watery grave and, over the next 8 years, restored to full active service. Sisters K92 and K94 have also been salvaged from the Oreti river. K92 has been restored to full active service, and has re-established her position on the Kingston Flyer train, which the K class made famous at the end of the 19th century.
- The character Sammy from Sammy the Shunter is based on the UK design of the engine.
- Two characters in the 1991 film The Little Engine That Could have this kind of wheel arrangement.
- Albert from The Railway Series has this wheel arrangement.
- Bertram Baxter, British Locomotive Catalogue 1825-1923, Vol. 2A, Moorland Publishing, 1978 ISBN 0-903485-51-6, p.48.