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Great Northern, Piccadilly & Brompton Railway (GNP&BR), of Hamilton House, Victoria Embankment, London
Later known as the Piccadilly Line.
One of several railways controlled by the Underground Electric Railways Co of London Ltd (UERL), whose chief director was Charles Yerkes, although he died before any of his schemes came to fruition. 
1897 The company was incorporated by the Brompton and Piccadilly Circus Railway Act of 1897. 
1902 there had been 26 Bills before Parliament to construct tube railways in London, many of them proposing competing routes and it required a Parliamentary Committee to decide on the most worthy of them as far as the Piccadilly line was concerned.
1902 The undertaking of the Great Northern and Strand Railway was transferred to this company.
The scheme eventually agreed involved the amalgamation of two of the planned tube railways, the Great Northern and Strand Railway (GN&SR) and the Brompton and Piccadilly Circus Railway (B&PCR), and the taking over of a District Railway scheme for a deep-level tube line between South Kensington and Earl's Court (approved in 1897 but not built). A connecting section between Piccadilly Circus and Holborn was also added to link the GN&SR and B&PCR.
1906 The works were carried out by the Underground Electric Railways Co; 8 1/8 miles of line were opened for traffic in December. When the GNP&BR was formally opened on 15 December 1906, the line ran from the Great Northern and City Line terminus at Finsbury Park to the District Railway's station at Hammersmith.
1907 On 30 November the short branch from Holborn to the Strand (later renamed Aldwych) opened. This had been planned as the last section of the GN&SR before the amalgamation with the B&PR was made; in 1905 (and again in 1965) plans were made to extend it the short distance south under the River Thames to Waterloo, but this was never came about. Although built with twin tunnels, single-line shuttle working became the norm from 1918, with the eastern tunnel closed to traffic.
1910 On 1 July the GNP&BR and the other UERL-owned railways (the Baker Street and Waterloo Railway, the Charing Cross, Euston and Hampstead Railway and the District Railway) were merged by private Act of Parliament to become the London Electric Railway Co.
1928 On 10 December a new Piccadilly Circus tube station, which included a sub-surface booking hall and 11 escalators, was opened. This was the start of a considerable development over the whole of the Railway, which included a comprehensive programme of station enlargement on the same basis as at Piccadilly Circus.
1933 Incorporated into the London Passenger Transport Board
The line has two depots, at Northfields and Cockfosters.
From the 1920s onwards there had been severe congestion at the line's northern terminus, Finsbury Park, where travellers had to change on to trams and buses for destinations in North and North East London. There had been deputations made to Parliament, asking for an early extension of the line either towards Tottenham and Edmonton or towards Wood Green and Palmers Green. The early 1930s was a time of recession, and in order to relieve unemployment Government capital was made available. The chief features of the scheme were an extension northwards from Finsbury Park to Cockfosters. It was also planned to build a station between Manor House and Turnpike Lane at the junction of Green Lanes and St Ann's Road in Harringay, but this was stopped by Frank Pick who felt that the bus and tram service at this point was adequate. However, a 'Ventilation Station', in similar architectural style to tube stations of the time was provided at the site, and is visible today. There was also some opposition from the London and North Eastern Railway to the line. The extension is in tube from Finsbury Park to a point a little south of Arnos Grove. The total length of the extension is 12 km (7.7 miles): it cost £4 million to build and was opened in sections as follows:
To Uxbridge: the District Railway had operated services to Uxbridge since 1910.
1919 A tunnelled link for the Central London Railway to the disused London and South Western Railway (L&SWR) tracks south of the L&SWR's Shepherd's Bush station was proposed. Although authorisation was granted in 1920, the connection was never realised, and the L&SWR tracks were eventually used by the Piccadilly Line when it was extended west of Hammersmith in 1932.
1932 The District services taken over by the Piccadilly line were:
At one time there was an intention to run Piccadilly line trains over the branch towards Richmond. The subways which were to carry these tracks are still visible from the eastbound lines on the approach to Turnham Green. This is why the existing westbound Piccadilly line track is carried above the level of the District line, though this is not necessary for the eastbound District lines trains from Richmond to reach the District main line.
During the planning stages of the Victoria line, a proposal was made to transfer Manor House station to the Victoria line, and also to build new "direct" tunnels from Finsbury Park to Turnpike Lane station, thereby cutting the journey time in and out of Central London. This idea was eventually shelved due to the inconvenience to passengers that would have been caused during re-building, as well as the costs of the new tunnels. Even so, the Piccadilly line was still affected at Finsbury Park by the construction of the Victoria line. The westbound service was re-directed through new tunnels, to give cross-platform interchange with the Victoria line on the platforms previously used by the Northern City Line. This work was completed in 1965, and the diversion came into use on 3 October 1965, three years before the opening of the first stage of the Victoria line.
1975 a new tunnel section was opened to Hatton Cross from Hounslow West. Hounslow West became a tunnel section station.
1977 the branch was extended to Heathrow Central.
1984 This station was renamed Heathrow Terminals 1, 2, 3, with the opening of a one-way loop serving Heathrow Terminal 4, to the south of the central terminal area.
2003 Administration changed to Transport for London
From 7 January 2005 to 17 September 2006, the loop via Heathrow Terminal 4 was closed to allow the connection of a spur line to the future Heathrow Terminal 5 station. All underground services reverted to two-way working into Terminals 1, 2 and 3, which again became the temporary terminus; shuttle buses served Terminal 4 from the Hatton Cross bus station. The station at Terminal 5 opened in 2008.