On 4th October 1911, the first moving staircase on the London Underground was demonstrated at Earl’s Court Station. Two escalators linked the Piccadilly Line platforms with the subway that runs under the District/Circle subsurface lines.
Services (including the District Line, coloured green on the Tube map) that run through tunnels just below the surface – built mostly by the “cut and cover” method – use trains and tunnels of a size comparable to those on British main lines.
The District Line runs westwards from Upminster, splitting into a number of branches at Earl’s Court. If you’re travelling to Richmond, say, you travel on the main route west towards Turnham Green, where, of the two further western branches, your train will take the one to Richmond.
The District Line is the only Underground line to cross the River Thames by bridge, one on the Wimbledon and one on the Richmond branch.