SR U1 class

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. 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. The similarity with GWR locomotive practice ended there, as a Midland Railway influence could be found in the placement of the locomotive fittings, with water top feed into the boiler located inside a dome, although a modification was made to the cab area, which was a variation of those featured on the N class and the 0 6 0s of Henry Fowler. 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. The new locomotives also varied with the increased amount of superheating surface area, and the outside valves were controlled by Walschaerts valve gear. The rebuilding process involved the removal of the side water tanks, the rear coal bunker and the trailing axles. In featuring similar components to the U class, the U1 was similar in profile, although a slab front was incorporated above the front buffer beam to house the third cylinder and associated valve gear assembly between the frames. Nevertheless, the U1 retained the 6 feet (2 m) driving wheels and general cab layout of the U class, and the Maunsell tender previously used on the N class locomotives, with No. A890 entering revenue earning service in June 1928.

No. A890 was the only U1 class member in operation as trials were undertaken to ascertain whether the design should be perpetuated instead of the N1 class for use on the more restricted routes of the Southern Railway. The success of the prototype U1 over the N1 design led to a construction order for a further 20 locomotives to be built at Eastleigh in 1931. Despite the possible weight savings with the Holcroft valve gear, the complexity of maintaining the linkage with the inside cylinder meant that the new class members were fitted with three separate sets of Walschaerts valve gear for all cylinders. As had happened with the N1 class prototype No. A822, No. A890 was modified to this revised specification in 1930 to reduce maintenance costs. Prior to the modification of No. 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 , although the slab front was retained. Further changes were made to the design, as No. A890 featured right hand drive, impeding forward vision for viewing signals from the footplate. The main batch was produced with left hand drive to resolve this problem. The main batch could also be distinguished by their higher running plates along the sides of the boiler, compared to the lower version on the rebuild that required larger wheel splashers to cover the tops of the driving wheels. 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. The trials made by the Southern Railway on the prototype decided the eventual route availability of the class so that they became a regular fixture on the restricted Tonbridge to Hastings route alongside their N1 class cousins. Their route availability also meant that they were used on Central section trains to Portsmouth and Southampton until replaced by V class locomotives. The U1 class worked troop trains along this route during the Second World War, proving their worth as rugged locomotives. 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. Crews praised their ability to achieve what they were designed to do, being thoroughly at home with the secondary duties that suited such a locomotive. The class was to be the final member of Maunsell's series of moguls, but the off cuts from the rebuilding of the K class and a similar boiler were used on yet another of the "family" of standardized locomotives, the 3 cylinder W class 2 6 4 heavy freight tank engine. All locomotives were inherited by British Railways in 1948, and from 1955, several members of the class received new chimneys of the British Railways Standard Class 4 variety and replacement cylinders, which had become worn through intensive use. 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. The U1 Class members working on the Hastings route were subjected to excess flange wear because of the tight curves, a problem also experienced when used on the network west of Exeter, when the aging T9 class 4 4 0 locomotives designed by Dugald Drummond were finally withdrawn from service in the early 1960s. 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. With the electrification of the Kentish main lines of the Eastern section, the amount of work that required the U1 class' route availability advantage was reduced, and the class was congregated at Stewarts Lane shed in London. The majority of the class was withdrawn from service in 1962, with the final three (including the prototype) gone by June 1963, outlasting their N1 class siblings by one year. Having been replaced by Bulleid's Light Pacifics, no members of the U1 class were preserved.

The entire class was painted in Maunsell's lined Southern Railway Olive Green with yellow markings and "Southern" on the tender. During the Second World War, the class was painted wartime black livery with Bulleid's "Sunshine Yellow" lettering. The U1s were initially numbered in the Southern Railway's post grouping series from A890–A900 for the first eleven constructed. 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. This helped to prevent confusion with locomotives of similar numbers that the Southern Railway acquired from other pre grouping companies. The "A" in this case denotes Ashford works. This was changed to the series 1890–1910 during the 1930s, when the final members of the class were constructed.

The class was absorbed by British Railways in 1948, and were given the British Railways Power classification 4P3F. Livery was initially similar to the Southern Railway, though with "British Railways" on the tender, and an 'S' prefix to the Southern number. This was succeeded by the British Railways mixed traffic lined black livery with red, cream and grey lining and the British Railways crest on the tender. The British Railways standard numbering system was used to replace the Southern Railway system, and the class was allocated the series 31890–31910.