The London Underground is a public transport network, composed of electrified railways (that is, a metro system) that run underground in tunnels in central London and above ground in the London suburbs. It is the oldest city underground network in the world. Lines on the Underground can be classified into two types: sub-surface and deep level. The sub-surface lines were dug by the cut-and-cover method, with the tracks running about 5 metres below the surface. The deep-level or "tube" lines, bored using a tunnelling shield, run about 20 metres below the surface (although this varies considerably), with each track running in a separate tunnel lined with cast-iron rings. Today there are 275 stations and over 408 km of active lines, with 3 million passenger journeys made each day. (more...)
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