Maunsell incorporated several improvements, notably to the steam circuit and the locomotive's loading gauge, allowing it to operate on routes with height and width restrictions.

The design was also to power traffic including milk trains, which required fast transit to the dairies in London.

The outline was made during the First World War, and incorporated lessons learned from the operation of his H15 class, a design that was to provide the basis for future standardisation on the LSWR.

Richard Maunsell was given the newly created post of CME to the Southern Railway, and decided to revise the cylinder arrangement of the locomotive.

[12] 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.

[13] Fifteen locomotives of this revised design were built in 1927, and some were given 4,000-imperial-gallon (18,000 L; 4,800 US gal) six-wheeled tenders for use on the Southern Railway's Central section.

[14] The standardisation measures undertaken by both Urie and Maunsell were soon vindicated by the fact that tenders and other parts were swapped with those of other classes on the Southern Railway when locomotives were under overhaul.

[1] A final modification was also applied to the class at this time, when all locomotives were equipped with smoke deflectors to improve visibility from the footplate when travelling at speed.

[1] Both Urie and Maunsell S15s spent most of their working lives on the Southern Railway's Western section, although they were sometimes used on inter-regional freights.

[16] In order to increase maintenance efficiency, all Urie S15 locomotives (which had the lower boiler pressure) were concentrated at the Southern Railway's London freight depot at Feltham.

[17] This yard also featured the Maunsell S15s, which were allocated to Exmouth Junction, Hither Green and Salisbury, demonstrating the "go anywhere" nature of the class.

[1] Despite the design being only a year newer, the S15s outlasted their N15 King Arthur class counterparts because of their dual freight/passenger abilities, though they were retired between 1962 and 1966 as part of the British Railways Modernisation Plan.

[15] Maunsell S15 number 30837 became the final member of the class in operation, returning to Feltham in January 1966 to work a farewell rail tour.

All the surviving members of the class were purchased from Woodham Brothers scrapyard in Barry, Vale of Glamorgan, South Wales.

[27] The only slight livery modification occurred before the Second World War, when Oliver Bulleid introduced the "Sunshine Yellow" lettering and numbering.