The LB&SCR D1 class were powerful 0-4-2 suburban passenger tank locomotives, designed by William Stroudley of the London Brighton and South Coast Railway in 1873. They were originally known as "D-tanks" but later reclassified as class D1. Members of this very successful class survived in service until 1951.

Under the Whyte notation for the classification of steam locomotives, 0-4-2 represents the wheel arrangement with no leading wheels, four powered and coupled driving wheels on two axles and two trailing wheels on one axle. While the first locomotives of this wheel arrangement were tender engines, the configuration was later often used for tank engines, which is noted by adding letter suffixes to the configuration, such as 0-4-2T for a conventional side-tank locomotive, 0-4-2ST for a saddle-tank locomotive, 0-4-2WT for a well-tank locomotive and 0-4-2RT for a rack-equipped tank locomotive.