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
History of the London Underground Harrow (later Uxbridge) and Hounslow. In 1933, the underground railways and all London area tram and bus operators were merged into the London Passenger Transport Board (LPTB). The outlying branches of the Metropolitan were closed; various upgrades were planned. The Bakerloo line's extension to take over the Metropolitan's Stanmore branch, and extensions of the Central and Northern lines, formed part of the 1930s New Works Programme. The outbreak of World War II in 1939 halted or interrupted some of this work, and many tube stations were used as air-raid shelters. The LPTB was nationalised in 1948, and the reconstruction of