SR U1 class

SR U1 class
Maunsell U1 class 2-6-0 Number A899 (Eastleigh 1931) at Waterloo. Note the slab-front – incorporated above the front buffer beam to house the inside cylinder – and the absence of smoke deflectors
Type and origin Power type Steam Designer Richard Maunsell Builder SR Ashford/Eastleigh Works Build date 1928–1931 Total produced 20 (+1 rebuilt from K1 class)
Specifications Configuration: ​  • Whyte 2-6-0  • UIC 1′C h3 Gauge 4 ft 8 1⁄2 in (1,435 mm) standard gauge Leading dia. 3 ft 1 in (0.940 m) Driver dia. 6 ft 0 in (1.829 m) Length 57 ft 10 in (17.63 m) Total weight 107 long tons 14 cwt (241,200 lb or 109.4 t) 109.4 t; 120.6 short tons Fuel type Coal Fuel capacity 5 long tons 0 cwt (11,200 lb or 5.1 t) 5.1 t; 5.6 short tons Water cap 4,000 imp gal (18,000 l; 4,800 US gal) Boiler pressure 200 lbf/in2 (1.38 MPa) Cylinders Three Cylinder size 16 in × 28 in (406 mm × 711 mm) Valve gear Walschaerts
Performance figures Tractive effort 25,387 lbf (112.93 kN)
Career Operators Southern Railway → British Railways (Southern Region) Class SR: U1 Power class SR: B BR: 4P3F Withdrawn 1962–1963 Disposition All scrapped

The SR U1 class were three-cylinder 2-6-0 ('mogul') steam locomotives designed by Richard Maunsell for passenger duties on the Southern Railway. The fifth member of the Maunsell "family" of standardised moguls and 2-6-4 locomotives, the U1 was the final development of the Maunsell mogul, and marked a continuation of the basic principles established by CME George Jackson Churchward for the GWR. Developed from Maunsell's previous SR U class design, the U1 class shared characteristics with Churchward’s GWR 4300 Class.

The U1 prototype was a rebuild of the unique 3-cylinder SR K1 ("River") class 2-6-4 tank locomotive, becoming operational in June 1928. The design was part of a drive to create a standard fleet of locomotives using parts interchangeable with other Maunsell-designed classes. The three smaller cylinders increased their route availability over the 2-cylinder U class. The K1 rebuild featured a variant of the Gresley conjugated valve gear, previously trialled on the SR N1 class prototype, designed by one of Maunsell’s assistants, ex-GWR engineer Harold Holcroft.

The class was to supersede the production of N1 class locomotives because of its good performance, which amounted to an order for 20 more U1 locomotives in 1931. Production was halted at 21 locomotives, the class gaining a good reputation amongst crews, and all were transferred to British Railways (BR) ownership following nationalisation in 1948. They continued to give valuable service until the Kentish main line electrification scheme was completed in the early 1960s. The entire U1 class was withdrawn from service by 1963, and none were preserved.