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
TfL Rail and in Essex. That branch comprises the first 14 stations on the Great Eastern Main Line, with interchange at Shenfield for medium- and long-distance services beyond to East Anglia. In May 2018 TfL Rail also took over operation of the Heathrow Connect service between in central London and Heathrow Airport to the west (Heathrow Terminal 4 station being the terminus). That route has nine stations. The service is operated by MTR Corporation under contract to Transport for London (TfL). Between May 2016 and May 2017, TfL Rail carried over 47 million passengers on the Shenfield branch. In June 2013 TfL