The London Underground S7 and S8 Stock, commonly referred to as S Stock, is a type of passenger train running on the London Underground's subsurface lines since 2010. Manufactured by Bombardier Transportation's Derby Litchurch Lane Works, the S Stock was ordered to replace the A60, A62, C69, C77 and D78 stock on the Metropolitan, District, Hammersmith & City, and Circle lines, which all dated from the 1960s and 1970s. The order was for a total of 192 trains (1,403 cars), and consisted of two types, S7 Stock for the Circle, District and Hammersmith & City lines, and S8 Stock for the Metropolitan line, with differences in the arrangement of seating and number of cars. Both types have air-conditioning and lower floors to ease accessibility for disabled people, and also have open gangways to allow passengers to move from one car to another whilst the train is moving. The order was said to be the biggest single rolling-stock order in Britain at, according to Transport for London, a cost of £1.5 billion. Passenger service began on the Metropolitan line in July 2010, the Hammersmith & City line in July 2012, and the Circle and District lines in September 2013. The S Stock completely replaced the A Stock on the Metropolitan line in September 2012, and the C Stock on the Circle and Hammersmith & City lines in February 2014, and on the Edgware Road branch of the District line in June 2014; it fully replaced the D Stock on the rest of the District line in April 2017. Overview The S designation stands for sub-surface following the London Underground tradition of designating surface stock with a letter associated with its intended route—A Stock on the Metropolitan line to Amersham, the C Stock on the Circle line, and D Stock on the District line. Part of Bombardier's Movia family, the stock consists of 133 S7 seven-car trains for the Circle, District and Hammersmith & City lines, and 59 S8 eight-car trains for the Metropolitan line. There were also three additional eight-car trains called an S7+1 in use on the Metropolitan Line while the eight-car S8 trains were returned for engineering modifications and ATC fitment. These trains were formed of a seven-car S7 and an additional car from another S7, forming an eight-car train. These trains, however, retained the all-longitudinal seating of the S7, and thus could be distinguished from the remaining S8s.