The SR U class were 2-6-0 (mogul) steam locomotives designed by Richard Maunsell for passenger duties on the Southern Railway (SR). The class represented the penultimate stage in the development of the Southern Railway’s mogul "family", which improved upon the basic principles established by GWR Chief Mechanical Engineer (CME) George Jackson Churchward for Great Western Railway (GWR) locomotives. The U class design drew from experience with the GWR 4300 and N classes, improved by applying Midland Railway ideas to the design, enabling the SECR to influence development of the 2-6-0 in Britain. The U class was designed in the mid-1920s for production at a time when more obsolete 4-4-0 locomotives were withdrawn, and derived from Maunsell’s earlier SECR K (“River”) class 2-6-4 tank locomotives. The first 20 members of the U class were rebuilds of the K class locomotives, one of which was involved in the Sevenoaks railway accident. A further 20 U class locomotives were built in 1928 to fill the gap in cross-country and semi-fast express passenger services after the withdrawal of the K class. The design also continued the standardisation of the Southern Railway locomotive fleet by using parts designed to be interchangeable with other Maunsell-designed classes.

A total of 50 locomotives were built over three batches between 1928 and 1931, and the design formed the basis for the 3-cylinder U1 class of 1928. They were able to operate over most of the Southern Railway network, gaining the nickname "U-Boats" after the submarine warfare of the First World War, and continued to operate with British Railways (BR). The class saw continuous use until 1966, when all members of the U class were withdrawn from service. Four U class locomotives have been preserved on two heritage railways in the south of England.