A deep-level Tube platform is typically about 18 m below the street; a sub-surface line, dug in a trench and roofed over, about 6 m.
In TfL’s table of platform levels the deep-level lines’ platforms below the street have a median depth of 18.4 m, and the Metropolitan, District, Circle and Hammersmith & City platforms 6.0 m. The deepest is Hampstead, 58.5 m, because it is under a hill.
Depth below the street depends on the hill above as much as the tunnel below: Hampstead is deep because its street is 108 m above sea level. Many deep-level stations in the suburbs are at the surface, and Greenford’s platform is 10.2 m above the street.
Estimate with the rule, then check it against the calculator that models it properly.
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