Deep level and Sub surface

The lines on the London Underground can classified into two types; deep level and sub surface.

Sub Surface
The subsurface lines were dug by the cut-and-cover method, with the tracks running about 5 m below the surface. Trains on the subsurface lines slightly exceed the standard British loading gauge.

Deep level
The deep-level or tube lines, bored using a tunnelling shield, run about 20 m below the surface (although this varies considerably), with each track in a separate tunnel lined with cast-iron or precast concrete rings. These tunnels can have a diameter as small as 3.56 m (11 ft 8.25 in) and the loading gauge is thus considerably smaller than on the subsurface lines. Hampstead is the deepest station under the surface at 58.5 metres (64.0 yards, 192 feet). However, it is not the deepest below sea level as the station's surface building is near the top of a hill. The deepest platforms below sea level are the Jubilee Line platforms at Westminster which are -32 metres (-35 yards).

Other Information
Lines of both types usually emerge onto the surface outside the central area, except the Victoria line, which is in tunnel except for its depot, and the very short Waterloo & City line, which runs entirely in the central area and has no surface section. Only 45% of the Underground is in tunnel. The highest point above ground on the Underground is the viaduct over Dollis Road between Finchley Central and Mill Hill East on the Northern Line. It is 18 metres (60 feet) above the ground.

While the tube lines are for the most part self-contained, the subsurface lines are part of an interconnected network: Each shares track with at least two other lines. The subsurface arrangement is somewhat similar to the New York City Subway, which also runs separate "lines" over shared tracks.