London Underground with smaller trains that run in two circular tunnels ("tubes") with a diameter about . These lines have the exclusive use of a pair of tracks, except for the Piccadilly line, which shares track with the District line between Acton Town and Hanger Lane Junction and with the Metropolitan line between Rayners Lane and Uxbridge; and the Bakerloo line, which shares track with London Overground services north of Queen's Park. Fifty-five per cent of the system runs on the surface. There are of cut-and-cover tunnel and of tube tunnel. Many of the central London underground stations on deep-level tube routes
London Underground S7 and S8 Stock be fully served by the S Stock, due to its existing D Stock being newer than other trains that the S Stock was ordered to replace, as well as its recent refurbishment. The first S7 Stock train entered passenger service on the line between Olympia and West Ham on 2 September 2013. On 6 February 2014, the S Stock started running between Wimbledon and Edgware Road. The S Stock started services to Ealing Broadway on 13 June 2014, and on 17 June saw the start of commercial service to Richmond. On 16 January 2015, the Stock began running to Upminster.