London Underground infrastructure

The infrastructure of the London Underground includes 11 lines, with 270 stations. There are two types of line on the London Underground: services that run on the sub-surface network that is just below the surface and use larger trains, or the deep-tube lines, that are for the most part self-contained and use smaller trains. Most of the lines emerge onto the surface outside the central area. The oldest running on the Underground was introduced in 1972. The Underground is electrified using a four-rail system, the DC traction supply being independent of the running rails. Planned improvements include new stations, line extensions and more lines with automatic train operation (ATO).

The total length of railway on London Underground is 250 miles (402 km) and made up of the sub-surface network and the deep-tube lines. In 1971/72 it was remeasured in kilometres using Ongar as the zero point.

The Circle, District, Hammersmith & City, and Metropolitan lines are services that run on the sub-surface network that has railway tunnels just below the surface and built mostly using the "cut-and-cover" method. The tunnels and trains are of a similar size to those on British main lines. The Hammersmith & City and Circle lines share all their stations and most of the track with other lines. The Bakerloo, Central, Jubilee, Northern, Piccadilly, Victoria and Waterloo & City lines are deep-level tubes, with smaller trains that run in two circular tunnels with a diameter of about 11 feet 8 inches (3.56 m), lined with cast-iron or precast concrete rings, which were bored using a tunnelling shield. It is these that were the tube lines, although since the 1950s the term "tube" has come to be used to refer to the whole London Underground system. Many of the central London deep-tube line stations, such as the Central and Piccadilly lines, are higher than the running lines to assist deceleration when arriving and acceleration when departing. The deep-tube lines generally 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 North Ealing 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.

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