London Underground consists of 11 lines, with a total of 402 kilometers (250 mi) in length, which makes the Underground being the fourth-longest metro system worldwide. Underground lines are made up of the deep-tube lines and the sub-surface network lines.
Around 55% of the line system runs on the surface. London Underground has 93 miles (150 km) of the tube tunnel and about 20 miles (32 km) of the cut-and-cover tunnel.
The Underground lines are electrified with a four-rail DC system. A conductor rail among the rails is energized at -210 V; Another rail outside the running tracks is excited at +420 V, which gives a difference of about 630 V in general.
The average speed on the Underground reaches 20.5 mph (33.0 km/h); While the outside tunnels of central London run at over 40 mph (64 km/h), mostly in the countryside and suburban areas. The Metropolitan line of London can reach the maximum speeds of 62 mph (100 km/h).
The Circle, Metropolitan, District, and Hammersmith & City lines are part of the sub-surface network. These lines have a circular bi-directional loop around zone 1. The Circle Line and the Hammersmith & City share stations, the most of their track, along with the district and the Metropolitan lines.
The Bakerloo, Jubilee, Northern, Central, Piccadilly, Waterloo & City, and Victoria lines are the part of the deep-level tube lines, with smaller trains that commute in two circular tunnels, or simply called tubes, with a diameter around 11 feet and 8 inches (3.56 m).
These lines use a pair of tracks, except for the Uxbridge branch on the Piccadilly Line, which shares the track with the District line between Hanger Lane Junction and Acton Town, and also with the Metropolitan line (sub-surface network line) between Uxbridge and Rayners Lane. Besides, it shares the tack with the Bakerloo line, along with London Overground's Watford DC Line, which has an aboveground section on the north of Queen's Park.
The London Underground uses several railways and alignments, those which were built by mainline railway companies. Below are the following lines:
Bakerloo line runs between Queen's Park and Harrow & Wealdstone and over the Watford DC Line, which is also used by London Overground, along with the London & North Western Railway (LNWR). The central line runs between Newbury Park and Woodford junction via Hainault.
The district line is on the South of Kensington Olympia. It runs between Campbell Road junction, Bromley-by-Bow, and Barking. District line has the westbound track between the east of Ravenscourt Park, Turnham Green, and Turnham Green to Richmond, which is also used by London Overground. Concerning the eastbound track is operates between Turnham Green and east of Ravenscourt Park.
The Hammersmith & City line departs between Westbourne Park Underground station and Paddington and, also the line runs alongside the mainline. The current two tracks are available to the north of the mainline, and the subway on the east of Westbourne Park opened in 1878. However, nowadays, the Hammersmith & City line is segregated from the mainline.
The Jubilee line route runs between Stratford and Canning Town. It was built in 1846 by the GER. The current Docklands Light Railway (formerly known as North London line) uses the original eastern alignment, while the Jubilee line uses the western alignment.
The Northern line from East Finchley to Mill Hill East opened in 1867. While from Finchley Central to High Barnet, it established in 1872s. The Great Northern Railway founded both of them.
The Piccadilly line has the westbound track between Turnham Green and the east of Ravenscourt Park, built-in 1869, by LSWR. The line was initially used for district and eastbound mainline services.