District Line
The District Line is the busiest sub-surface line (and third busiest overall) on the London Underground, carrying some 188,000 journeys per year, and is represented on the London Tube Map by a green line. It serves 60 stations and is roughly 40 miles long.
Originally built by the Metropolitan District Railway, the District Line first opened in 1868 and was expanded. The company was brought in as part of the 'Underground Group' by Charles Yerkes and was nationalised sometime later, in the 1930s. The original line has branches to Hounslow West and Uxbridge (now on the Piccadilly line) and as far as Southend-on-Sea as well as Windsor to Ealing Broadway on the Great Western Main Line.
Image from the District Line Article on Wikipedia.
Most of the trains on the District Line are D Stock, but the Edgware Road to Wimbledon uses C Stock since the platforms between Kensington High Street and Edgware Road are shorter. The D Stock has been refurbished and maintained and has a patriotic red, white and blue livery.
The carriages were also upgraded with completely new interiors and CCTV, plus passenger information displays, all maintained out of the Upminster and Ealing Common depots.
The District Line is the only line on the tube to traverse the Thames on a bridge, rather than a tunnel. Additionally, the last silver train on the London Tube was unit 7115 (Again, D Stock) which was repainted and refurbished in Feb 2008 - on the District Line.
The Line also features on the popular UK Soap, Eastenders as its local tube stop is on the District Line. Film scenes featuring District Line trains are used on the programme, shown running into the fictional Walford East station.