London: District Line, Green
The District line is a line of the London Underground, coloured green on the Tube map. It is a “sub-surface” line, running through the central area in shallow cut-and-cover tunnels. It is the busiest of the sub-surface lines and the third busiest overall on the Underground network.
The District line was built by the Metropolitan District Railway (MDR) and opened in stages from 1868.
Source: Wikipedia.
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