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 most modern standards". The transit time from Brent Cross to Surbiton was quoted as 28 minutes, with a maximum train speed of 80 km/h. The promoters cited a number of reasons why they believed an underground scheme would be cost-effective at £1.75bn: with lower tunnelling costs as the tunnel diameter would be smaller than for a heavy rail scheme; there are no other rail tunnels to avoid (as in central London); and the subsoil strata are suitable for modern tunnel boring machines (TBMs). A number of other notable orbital rail schemes for London have been previously proposed: West London Orbital