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 of London and especially the Blitz led to the use of many tube stations as air-raid shelters, with 175,000 people arriving every night in August 1940. Six stations were breached by a direct hit, and in March 1943, 173 people died in a crowd crush accident at the unfinished Bethnal Green station. In the 1940s a depot built for the Northern line extension and an unfinished stretch of the Central line extension, the underground section between Newbury Park and Leytonstone, was turned into aircraft factory. The closed Brompton Road station was used as an anti-aircraft control room. The closed Down