[27] Both railways expanded, the District building five branches to the west reaching Ealing, Hounslow,[28] Uxbridge,[29] Richmond and Wimbledon[28] and the Metropolitan eventually extended as far as Verney Junction in Buckinghamshire – more than 50 miles (80 km) from Baker Street and the centre of London.

[30] For the first deep-level tube line, the City and South London Railway, two 10 feet 2 inches (3.10 m) diameter circular tunnels were dug between King William Street (close to today's Monument station) and Stockwell, under the roads to avoid the need for agreement with owners of property on the surface.

[38] In the early 20th century, the District and Metropolitan railways needed to electrify and a joint committee recommended an AC system, the two companies co-operating because of the shared ownership of the inner circle.

[47] After the war, government-backed financial guarantees were used to expand the network and the tunnels of the City and South London and Hampstead railways were linked at Euston and Kennington;[48] the combined service was not named the Northern line until later.

[61] They were not always a guarantee of safety however; on 11 January 1941 during the London Blitz, a bomb penetrated the booking hall of Bank Station, the blast from which killed 111 people, many of whom were sleeping in passageways and on platforms.

[62] On 3 March 1943, a test of the air-raid warning sirens, together with the firing of a new type of anti-aircraft rocket, resulted in a crush of people attempting to take shelter in Bethnal Green Underground station.

A total of 173 people, including 62 children, died, making this both the worst civilian disaster in Britain during the Second World War, and the largest loss of life in a single incident on the London Underground network.

[64][65][66] Under the same act, the country's main line railways were also nationalised, and their reconstruction was given priority over the maintenance of the Underground and most of the unfinished plans of the pre-war New Works Programme were shelved or postponed.

[1] Many of the central London Underground stations on deep-level tube routes are higher than the running lines to assist deceleration when arriving and acceleration when departing.

[136] The long proposed Croxley Rail Link (an extension of the Metropolitan line) was cancelled in 2018 due to higher than expected costs and lack of funding.

Although relatively modern and well within their design life, the trains need work in the medium term to ensure the continued reliability of the traction control equipment and maintain fleet serviceability until renewal, which is expected between 2028 and 2032.

[154][155] On 9 October 2014, TFL published a shortlist of those (Alstom, Siemens, Hitachi, CAF and Bombardier) who had expressed an interest in supplying 250 trains for between £1.0billion and £2.5billion, and on the same day opened an exhibition with a design by PriestmanGoode.

[165] A 2000 study reported that air quality was 73 times worse than at street level, with a passenger inhaling the same mass of particulates during a twenty-minute journey on the Northern line as when smoking a cigarette.

[129][171][172] In the original Tube design, trains passing through close fitting tunnels act as pistons to create air pressure gradients between stations.

This system depends on adequate cross-sectional area of the airspace above the passengers' heads in the foot tunnels and escalators, where laminar airflow is proportional to the fourth power of the radius, the Hagen–Poiseuille equation.

An example is Green Park tube station, where false ceiling panels attached to metal frames have been installed that reduce the above-head airspace diameter by more than half in many parts.

Now, the ventilation space above head height is crowded with ducting, conduits, cameras, speakers and equipment acting as a baffle plates with predictable reductions in flow.

In January 2019, for example, it reported that pollution from particulates is up to 30 times higher on the London Underground than on streets in the roads above, with the Northern Line having the worst air quality.

[187] In mid-2012, London Underground, in partnership with Virgin Media, trialled Wi-Fi hotspots in many stations, but not in the tunnels, that allowed passengers free internet access.

[220] Called a "Freedom Pass", it allows free travel on TfL-operated routes at all times and is valid on some National Rail services within London at weekends and after 09:30 on Monday to Friday.

[225][226] The nightly closures are used for maintenance,[225] but some lines stay open on New Year's Eve[227] and run for longer hours during major public events such as the 2012 London Olympics.

Meanwhile, Mayor of London Boris Johnson decided it should be demolished along with the Earls Court Exhibition Centre as part of Europe's biggest regeneration scheme.

[203] The tube map came second in a BBC and London Transport Museum poll asking for a favourite UK design icon of the 20th century[274] and the underground's 150th anniversary was celebrated by a Google Doodle on the search engine.

[291] The Metropolitan Railway's original seven stations were inspired by Italianate designs, with the platforms lit by daylight from above and by gas lights in large glass globes.

[293] The City & South London Railway opened with red-brick buildings, designed by Thomas Phillips Figgis, topped with a lead-covered dome that contained the lift mechanism and weather vane (still visible at many stations, such as Clapham Common).

[295] In the first decade of the 20th century Leslie Green established a house style for the tube stations built by the UERL, which were clad in ox-blood faience blocks.

[299] Harry W. Ford was responsible for the design of at least 17 UERL and District Railway stations, including Barons Court and Embankment, and claimed to have first thought of enlarging the U and D in the UNDERGROUND wordmark.

[341][user-generated source] In 2016, British composer Daniel Liam Glyn released his concept album Changing Stations based on the 11 main tube lines of the London Underground network.

[348] Analysis of the Underground as a network may also be helpful for setting safety priorities, since the stations targeted in the 2005 London bombings were amongst the most effective for disrupting the transportation system.

[349] A study in March 2023, showed that over £1.3 million worth of mobile phones were stolen on the London Underground in 2022, more than the entire UK rail network combined.