The District line is a London Underground service that crosses Greater London from east to west. From Upminster, the eastern terminus, the line runs through Central London to Earl's Court before dividing into three western branches, to Ealing Broadway, Wimbledon and Richmond. There is a short branch that goes from Earl's Court to Kensington (Olympia). A branch also runs north from Earl's Court to Edgware Road via Paddington. Coloured green on the tube map, the line serves 60 stations in 40 miles (64 km), and with bridges across the Thames on the Wimbledon and Richmond branches is the only London Underground line to cross the river in this way. The track and stations between Barking and Aldgate East are shared with the Hammersmith & City line, and between Tower Hill and Gloucester Road and on the Edgware Road branch with the Circle line. Some of the stations are shared with the Piccadilly line. Unlike London's deep-level tube railways, the railway tunnels are just below the surface, and the trains are of a similar size to those on British main lines.
The District line is the busiest of the sub-surface lines as well the fifth busiest line overall on the London Underground with over 208 million passengers in the year 2011/12. The original Metropolitan District Railway (as it was then called) opened in December 1868 from South Kensington to Westminster as part of a plan for an underground 'inner circle' connecting London's main-line termini. Services were operated at first using wooden carriages hauled by steam locomotives. Electrification was financed by the American Charles Yerkes, and electric services began in 1905. In 1933 the railway was absorbed by the London Passenger Transport Board. In the first half of the 1930s the Piccadilly line took over the Uxbridge and Hounslow branches, although a peak-hour District line service ran on the Hounslow branch until 1964. Kensington (Olympia) has been served by the District line since 1946, and a short branch to South Acton closed in 1959. The trains carried guards until one-person operation was introduced in 1985. The signalling system is being upgraded, and the current D Stock trains are to be replaced by new 7-car S Stock trains by the end of 2016. wikipedia