basis for future standardisation on the LSWR. To economise on maintenance, the S15 class had interchangeable components that could be used on a similar design, the N15 class passenger locomotive, which had the same overall appearance. Details such as the boiler, the two-cylinders and valve gear were standardised between the classes, although a taper boiler was used on the S15 and N15 classes, as opposed to the parallel boiler of the H15 class. The only other major difference was the smaller diameter of the driving wheels. Smaller diameter wheels gave better traction, essential for a fast freight locomotive. By May 1921, sixteen were in service, and were mostly allocated to the London area, including the new marshalling yard at Feltham. Despite being mechanically sound, changes were made to Urie's original design after the Grouping of railway companies in 1923. Urie retired as Locomotive Superintendent when the LSWR was amalgamated into the Southern Railway in 1923. Richard Maunsell was given the newly created post of CME to the Southern Railway, and decided to revise the cylinder arrangement of the locomotive. In doing so, he delayed the construction of further locomotives until the modifications had been made. When the modifications were trialled in service, it became evident amongst locomotive crews that Maunsell had taken a sound design and made it better, achieving a consistent locomotive capable of undertaking all the tasks for which it was intended. With the successful implementation of the modifications, permission was given by the management of the Southern Railway for a second batch of locomotives to be constructed. In November 1941 during World War II, Nos. 496-499 were loaned to the Great Western Railway to assist with increased freight traffic on that railway. They were returned to the SR between March and July 1943. Second batch – Maunsell 1927–28 Maunsell's modifications included increasing the boiler pressure from to , and the reduction of the cylinder bore by half an inch. The footplate was also modified for operation on the Southern's new composite loading gauge, and differed from previous batches in having the Ashford-style cab, which was usually fitted to LBSCR locomotives. Unlike the original Drummond cab that was also favoured by Urie, the Ashford-style cab was of an all-steel construction and had a roof that was flush with the cab sides. It was inspired by the standard cab developed in 1904 by R. M. Deeley for the Midland