The central area contains stations close to tourist attractions, such as the London Transport Museum, Harrods, Buckingham Palace and Piccadilly Circus.

[7][8] The Heathrow branch remains at surface level until the eastern approach to Hounslow West station, where it enters a cut-and-cover tunnel.

A section of the DR's scheme for a deep-level tube line between South Kensington and Earl's Court was also added in order to complete the route.

[17] On 30 November 1907, the short branch from Holborn to the Strand (later renamed Aldwych) opened; it had been planned as the last section of the GN&SR before the amalgamation with the B&PCR.

[note 2] The Underground railways still suffered financial issues,[23] and to address this, the London Passenger Transport Board was established on 1 July 1933.

[34][35] In 1902, as part of an agreement for taking over the GN&SR, the Great Northern Railway (GNR) imposed a sanction on Yerkes to abandon the section north of Finsbury Park and they would construct the terminus below ground.

The London and North Eastern Railway (LNER), being the successor of the GNR, was placed in the position of electrifying its own services or withdrawing its veto of an extension of the Piccadilly line.

The extension would pass through Manor House, Wood Green and Southgate, ending at Enfield West (now Oakwood);[46][note 7] based on the absence of property development along the line.

Furthermore, since journey speed was a primary consideration in decision-making for the extension, the pencilled-in and much lobbied-for additional station near the corner of Green Lanes and St. Ann's Road in Harringay was dropped.

[50][44] Tunnel rings, cabling and concrete were produced in Northern England, while unemployed industrial workers there helped in the construction of the extension.

Construction of the extension started quickly, with the boring of the twin tube tunnels between Arnos Grove and Finsbury Park proceeding at the rate of a mile per month.

[65] The LER proposed an extension in November 1912 to Richmond due to available capacity to the west and the fact that passenger interchanges were large at Hammersmith.

[70][note 11] By 1925, the District line was running out of capacity west of Hammersmith, where services were headed to South Harrow, Hounslow Barracks, Richmond and Ealing Broadway.

At King's Cross St Pancras, the Piccadilly and Northern lines were finally connected via new escalators, albeit with construction delayed due to financial difficulties.

[91] On 13 October 1940, a bomb explosion caused the westbound platform tunnel at Bounds Green station to collapse, killing nineteen shelterers.

[100] Partial government funding was obtained in April 1972 for the 3.5 mi (5.6 km) Piccadilly line extension, and the estimated cost of construction was £12.3 million.

[101] On 27 April 1971, a construction ceremony was launched by Sir Desmond Plummer, leader of the Greater London Council, by bulldozing "the first sod".

[91] In the years after the Second World War, a series of preliminary plans for relieving congestion on the London Underground had considered various east–west routes through the Aldwych area, although other priorities meant that these were never proceeded with.

[129] Shuttle services were introduced between Hyde Park Corner and the Heathrow loop, between Acton Town and Rayners Lane, and between Arnos Grove and Cockfosters.

[131] This consisted of two-storey steel-framed buildings faced with dark oxblood red glazed terracotta blocks, with wide semi-circular windows on the upper floor.

Southgate was distinctively different, with a round base carrying a cylindrical panel of clerestory windows, topped by an illuminated feature with a bronze ball.

Green Park received a new shelter at the southern entrance; Piccadilly Circus had its ticket hall moved below street level.

[152][153][note 18] Green's stations such as Caledonian Road have bands of tiles arching overhead on the curved platform ceilings and above the tracks spaced 11–12 ft (3.4–3.7 m) apart.

Arnos Grove, Acton Town, South Harrow, Uxbridge, Hammersmith and Heathrow Terminal 5 have more than one siding for reversing trains or storing them.

[165] Stations such as Green Park and King's Cross St Pancras were installed with new lifts to provide step-free access to every platform by the 2012 Summer Olympics.

[195][196] Changes included the removal of transverse seating, strap hangers replaced with grab bars, new floor material and a full repaint into London Underground's corporate livery.

[204] In the mid 2010s, TfL began a process of ordering new rolling stock to replace trains on the Piccadilly, Central, Bakerloo and Waterloo & City lines.

[206] These trains will have an open gangway design, wider doorways, air conditioning and the ability to run automatically with a new signalling system.

[255] LUL then invited Alstom, Bombardier and Siemens Mobility to develop a new concept of lightweight, low-energy, semi-articulated train for the deep-level lines, provisionally called "Evo" (for 'evolution').

[257] Siemens Mobility was awarded a £1.5 billion contract in June 2018 to produce the new trains at a planned factory in Goole, East Yorkshire.