London Underground operate nearly 400 route miles of underground railway line, the majority of which is double track. Perhaps surprisingly, less than 50% of this is actually underground, a statistic which owes much to the system's expansion during the 1930's and 1940's when the network expanded into the suburbs over the branch lines of the mainline railway companies. Most of the underground lines are deep level tubes which currently extend for over 140 route miles with just 32 miles of sub-surface lines built by the original "cut and cover" method.
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