Under the Whyte notation for the classification of steam locomotives, 2-4-0 represents the wheel arrangement of two leading wheels on one axle, four powered and coupled driving wheels on two axles, and no trailing wheels.

The notation 2-4-0T indicates a tank locomotive of this wheel arrangement (its water is carried in tanks mounted on the locomotive, rather than in an attached tender).

Other equivalent classifications are:

Because of its popularity for a period with English railways, noted railway author C Hamilton Ellis considered the 2-4-0 designation to have the nickname (under the Whyte notation) of 'Old English'.

The 2-4-0 configuration was developed in the late 1830s or early 1840s as an enlargement of the 2-2-0 and 2-2-2 types, giving better adhesion. The type was initially designed for freight haulage.

The type was used on the Liverpool and Manchester Railway, the Birmingham and Gloucester Railway, the North Midland Railway and the London and South Western Railway (LSWR) before 1846. One of the earliest examples was the broad-gauge GWR Leo Class, designed by Daniel Gooch and built by R and W Hawthorn and Company, Fenton, Murray and Jackson, and Rothwell and Company during 1841 and 1842.