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
West London Orbital sections: The Dudding Hill Line, the North London Line and the Hounslow Loop. The Dudding Hill line itself has had no scheduled passenger service for over a century. It has no stations, no electrification and a 30 miles per hour (48 km/h) speed limit with semaphore signalling, and is lightly used by freight and very occasional passenger charter trains. It is roughly 4 miles (6.4 km) long. Near the site of Old Oak Common the West London Orbital would connect to the existing North London Line, and then further south at Acton, use the link down to the Hounslow Loop