The District Line is a line of the London Underground, the metro system in London, United Kingdom. The District is a 'sub surface line', meaning that the tunnel sections are close to the surface, being built by the cut and cover method. This is as opposed to the deep tube lines such as the Northern Line, which are bored at greater depth and use a smaller loading gauge. The District Line is the second longest line on the underground after the Metropolitan Line. It is also the busiest of the sub-surface lines, and the third busiest line of the London Underground overall. It is also well used by tourists as it links the tourist sights of the Natural History Museum, Westminster, and the Tower of London.
The main backbone of the District Line is the route running from Ealing Broadway on the Great Western Main Line in West London through the city to Upminster on the London, Tilbury and Southend Line. Upminster is on the eastern outskirts of London, very close to the Essex border, and is the third most distan terminus of the Underground after Chesham on the Metropolitan and Epping on the Central Line. The District Line also includes several branch routes. The longest of these is the one to Wimbledon, via Brompton, Fulham, and Putney. There are also branches to Richmond via Gunnersbury and Kew (sharing rails with the London Overground), Kensington Olympia (an infrequent shuttle servic e with no intermediate stops) and Edgware Road via Kensington, Notting Hill Gate and Bayswater(by way of running District trains over part of the Circle Line). The line has the most stations of any line (60) due to the many branches.
The District Line is mostly operated by purpose-built D-Stock trains (D standing for 'District'). These were built between 1978 and 1981 and were completely refurbished between 2005 and 2008. Due to short platforms on the Wimbledon branch, services on the route between Wimbledon and Edgeware Road are operated by C-Stock trains built for the Circle and Hammersmith and City Lines. These trains were built in 1968 and 1974. Both types are electric multiple units powered by 660V DC from a fourth rail system. Where tracks are shared with the Overground, a compromise voltage is used between this and the Overground's 750 DC third rail system. Incidentally, the D-Stock trains have single-leaf doors, which is now unique on the underground following the withdrawal of the contemporary 1983-stock tube trains. This is a disadvantage that causes longer loading and waiting times at stations. By 2015, both types of train will be replaced by new S-Stock trains (S for sub-surface). These trains will be articulated, making them the first modern underground trains where it will be possible to easily walk between carriages.