The SR U1 class were three-cylinder 2-6-0 ('mogul') steam locomotives designed by Richard Maunsell for passenger duties on the Southern Railway.

[4] 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.

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.

The Southern Railway had inherited routes with restricted loading gauge where the large, overhanging cylinders of a 2-cylinder locomotive could foul the walls of bridges and tunnels.

[11] The retention of the cylinder arrangement and valve gear lowered conversion costs, and meant that the locomotive could readily meet the route availability specification.

[12] Because of Holcroft's position as one of Maunsell's assistants, the new locomotive displayed many Swindon influences, making them similar to Churchward's 4300 class.

The Swindon ideas had also been used on the N, K, N1, U and K1 class designs, which included the Belpaire firebox and conical boilers, constructed at the North British Locomotive Works in Glasgow.

[13] The aesthetic aspects of the locomotive were contributed by another of Maunsell's assistants, James Clayton, who had moved to the SECR from the Midland Railway.

[9] The rebuilding process involved the removal of the side water tanks, the rear coal bunker and the trailing axles.

A890, the new locomotives were visibly different by the absence of the outside-to-inside valve linkage assembly and were given larger tenders with a water capacity of 4,000-imperial-gallon (18,184 L; 4,803.8 US gal), although the slab-front was retained.

[16] The main batch had also dispensed with the N class/Midland Railway-style double spectacle plates (the small windows on the front face of the cab) that was retained on the prototype, replacing them with one each side of the boiler.

[5] The main batch saw the front steps relocated ahead of the cylinders, as opposed to behind on the A890 rebuild, which was another relic of its previous guise as a 2-6-4 tank locomotive.

[16] The smokebox snifting valves were removed by Oliver Bulleid by the end of the Second World War, although the U1 class chimney was used to improve draughting on the other Maunsell moguls.

However, a trial on the Somerset and Dorset proved that the design was not good on steep gradients, and was thus confined to the Central and Eastern sections of the Southern Railway network.

[18] No major redesign of the front-end of the U1 class was required, with all the steam passages fully streamlined as a result of experience with the previous five Maunsell designs.

[16] The locomotive crews of the area took a dislike to the U1 in preference to the smaller-wheeled N class, which performed better on the steeper gradients found in this part of the railway network.

[1] The letter "A" reflected the post-grouping practice of the Southern Railway to denote the engine's place of origin within the pre-grouping (SECR in this case) number.