[2] The main route continues west from Earl's Court to Turnham Green after which it divides again into two western branches, to Richmond and Ealing Broadway.

It is the only Underground line to use a bridge to traverse the River Thames, crossing on both the Wimbledon and Richmond branches.

[5] 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 a below-ground "inner circle" connecting London's main line termini.

Kensington (Olympia) has been served by the District line since 1946, and a short branch to South Acton closed in 1959.

The first line opened in December 1868, with services from South Kensington to Westminster; these were operated by the Metropolitan Railway using wooden carriages hauled by steam locomotives.

By 1871, when the District began operating its own trains, the railway had extended to West Brompton and a terminus at Mansion House.

[7] Between 1 March 1883 and 30 September 1885, the District Railway ran trains between Mansion House and Windsor, via Paddington.

As part of the project that completed the Circle line in October 1884, the District began to serve Whitechapel.

[10] Services began running to Upminster in 1902, after a link to the London, Tilbury & Southend Railway (LT&SR) had been built.

[16] On 1 July 1933, the District Railway amalgamated with other Underground railways, tramway companies and bus operators to form the London Passenger Transport Board, and from 23 October 1933 Piccadilly line trains ran through to Uxbridge and the District line shuttle was withdrawn.

The 1935–1940 New Works Programme saw the Q Stock formed from these motor cars, upgraded with electro-pneumatic brakes and guard controlled air-operated doors, and the trailers replaced with new vehicles.

[24] The South Acton shuttle was withdrawn on 28 February 1959, followed by the peak hour District line through service to Hounslow on 9 October 1964.

[25] The whole District line service could not run through Aldgate East as this station was also served by Hammersmith & City trains, so some trains terminated at a bay platform at Mansion House, leaving the line east to Tower Hill overcrowded.

[28] A shorter train was needed on the Edgware Road branch due to the platform lengths so more of the C stock units, then already in use on the Circle and Hammersmith and City lines, were built.

[28] Driver-only operation of the trains was proposed in 1972, but due to conflict with the trade unions was not introduced on the District line until 1985.

[36] East of Earl's Court there is a grade-separated junction off the main line to the Edgware Road branch.

After Paddington station this branch joins the Hammersmith & City line at Praed Street junction, before terminating at the four-platform Edgware Road.

However, due to the nature of sub-surface lines, the cutting is occasionally left open both at and between stations for ventilation.

West of Earls Court, the line is entirely surface level, with the exception of the Hammersmith and Fulham Broadway stations, which are in cuttings built over by recent developments.

[51] Traction voltage was increased in 2017 from nominal 630 V to 750 V to give better performance and allow the trains to return more energy to the network through their regenerative brakes.

It was planned that a new signalling system would be used first on the sub-surface lines from the end of 2016,[54] but signalling contractor Bombardier was released from its contract by agreement in December 2013 amid heavy criticism of the procurement process[55] and London Underground subsequently awarded the contract for the project to Thales in August 2015.

[56] With the introduction of S7 Stock, the track, electrical supply, and signalling systems are being upgraded in a programme planned to increase peak-hour capacity on the line by 27 per cent by the end of 2023.

[8] Also now served by the Piccadilly line, the South Harrow branch opened from north of Ealing Common on 28 June 1903 and extended over Metropolitan Railway tracks to Uxbridge on 1 March 1910.

In the Sherlock episode "The Empty Hearse", a fictional unopened terminus station called Sumatra Road (situated underneath the Houses of Parliament as a disused branch line from Westminster Station) was created for the episode's story of a terrorism plot.