Printed in magenta on the tube map, the line is 41.4 miles (66.7 km) in length and serves 34 stations (13 of which are step free to platform).

Between Aldgate and Finchley Road, the track is mostly in shallow "cut and cover" tunnels, apart from short sections at Barbican and Farringdon stations.

It is the only Underground line with an express service at peak times; the resulting longer distance between stations means trains can achieve the system's highest speeds of up to 62 miles per hour (100 km/h) on some sections.

It first ran between Paddington and Farringdon, with wooden carriages and steam locomotives, subsequently extending north-westwards into the Middlesex countryside, where it stimulated the development of new suburbs.

Harrow was reached in 1880, and ultimately the line continued as far as Verney Junction in Buckinghamshire, more than 50 miles (80 km) from Baker Street.

The world's first underground railway, it opened on 10 January 1863 with gas-lit wooden carriages hauled by steam locomotives.

It reached Hammersmith in 1864 and Richmond in 1877; it completed the Inner Circle in 1884,[8] but the most important route became the line west and north-west into the Middlesex countryside, where it stimulated the development of new suburbs.

[10] In 1910, a seventeen-minute silent film was made showing large portions of the journey from Baker Street to Aylesbury and Uxbridge, seen from the cab of a train.

[12] On 1 July 1933, the Metropolitan was amalgamated with other Underground railways, tramway companies and bus operators to form the London Passenger Transport Board, and a period of rationalisation followed.

Goods services were passed to the London and North Eastern Railway, which also took over the role of providing steam locomotives for trains beyond the end of electrification at Rickmansworth.

[14] The 1930s was a period of rapid growth for the north-western suburbs of London, and LPTB developed ambitious plans to simplify the Metropolitan line and expand capacity.

[18] In the 1950s, F Stock trains, with sliding doors under the control of the guard, were transferred from the District line; these mainly worked the semi-fast Harrow and Uxbridge services.

[19] A major rebuilding of the main line from Wembley Park to Amersham was planned in the 1930s but delayed by the war.

The line from Wembley to Harrow was rebuilt immediately after the war and the project was completed from 1956 to 1962, on a more modest scale than originally planned.

The rebuilding electrified the line from Rickmansworth to Amersham, transferring all Aylesbury services to British Railways.

[22] One person operation of the trains was proposed in 1972, but due to conflict with the trade unions was not introduced on the line until 1986.

From just after Finchley Road, these four tracks run parallel with the Network Rail Chiltern Main Line from Marylebone.

[36] The central slow lines diverge here at a grade-separated junction to become the 7.5 miles (12.1 km) Uxbridge branch.

[36][39] The Watford branch has a triangle junction with the main line, with a north-facing curve between Croxley and Rickmansworth.

At Chalfont & Latimer, the 3.89 miles (6.26 km) single-track Chesham branch diverges, running parallel to the main line for a distance.

[44] Depending on the stopping pattern, services are advertised as either all-stations, semi-fast or fast on platform information boards and on-train announcements.

[45] No Metropolitan line trains call at any intermediate stations between Finchley Road and Wembley Park.

It was planned that a new signalling system would be used first on the line north of Baker Street from the end of 2016,[63] but signalling contractor Bombardier was released from its contract by agreement in December 2013 amid heavy criticism of the procurement process[64] and London Underground subsequently awarded the contract for the project to Thales in August 2015.

[65] With the introduction of S8 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.

[63][68] Trackside signals with automatic train protection (ATP) will remain on the line north of Harrow-on-the-Hill, shared with Chiltern Railways DMUs.

[17] Initially Verney Junction and Quainton Road remained open, with main line services provided by the LNER.

In 1939, the Stanmore branch and the stopping service between Finchley Road and Wembley Park were transferred to the Bakerloo line.

[16] In 1961, when steam locomotives were replaced and the line was electrified to Amersham, the Underground service to Great Missenden, Wendover, Stoke Mandeville and Aylesbury was withdrawn.

This route, never used for passenger traffic, continued to be used by London Transport's small fleet of steam engines until 1971, when diesels replaced them.

The branch closed some time after this, although a section of the spur line remained visible from a passing train.