The pannier tanks of the Great Western Railway are probably the best known of all British tank engines. First introduced in 1904 as rebuilds of the 2021 class saddletanks, the type became the mainstay of shunting, light freight and local passenger work in the GWR system.

Mr C. B. Collett introduced the 57XX class in 1929 and it was to become one of the largest in Britain, with a total of 863 delivered by 1950. 7715 itself was one of a batch of twenty-five built by Kerr Stuart and Co. of Stoke-on-Trent in 1930, shortly before the firm was taken over by the Hunslet Engine Company. The locomotive was Kerr Stuart works number 4450.

St Blazey, near Par, Cornwall, was to be the home of 7715 for the major part of its life on the national system. Duties included pick-up freight and the china clay traffic for which the area is well known. Work on these trains would have taken the engine to Bodmin and it is quite possible that 7715 may have met our Beattie Well Tank 0314 at this time. The locomotive's final years on the national network were spent at Duffryn Yard, Port Talbot.

Between 1958 and 1963, a number of 57XX class engines were transferred to London Transport. They were used on works trains and for shunting at Neasden and Lille Bridge Depots. 7715 was one of the last to arrive, in June 1963, and was given the number L.99 when it received the LT maroon livery (shown in the first picture above).

L.99 arrived at Quainton by road from Neasden on 2nd January 1970 and was used at the Easter open days of that year. It was then taken out of service for overhaul, returning to operation in Easter 1972, with its original GWR 7715 numberplates, though the engine was painted in a livery of black with the British railways lion emblem on the tank.  7715 subsequently received a heavy overhaul in 1979 and has since been painted in BR black and GWR green 1947 livery while working the Centre's trains. It then received another overhaul, which brought it up to British Rail mainline standard so that, as L99, it could work special steam trains on the London Underground from Neasden. 

During Winter 2003 it had an intermediate overhaul, which included a cylinder re-bore, with the assistance of the Severn Valley Railway  It is likely that this will be the last possible re-bore, before it will be necessary to sleeve the cylinders.  No. 7715 is currently outshopped in GWR green, and had a boiler ticket valid until March 2009.  After a failure at Xmas 2005 the loco spent a period out of operation until an agreement was reached with South Coast Steam for them to restore the locomotive.   Now back in operation it will be available for hire by South Coast Steam to other steam railways, and when not on hire returned to Quainton.  In July 2008 it was dispatched on hire to the Spa Valley Railway initially for one year.