Piccadilly line

One of the ventilation panels at Wood Green station platforms.

While early plans to serve Wood Green (specifically Alexandra Palace) existed since the 1890s as part of the GN&SR,[32][33] this section to Finsbury Park was later dropped from the GNP&BR proposal in 1902 when the GN&SR was merged with the B&PCR.[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.[35][36][37][38] Finsbury Park remained as an overcrowded terminus of the line, and was described as "intolerable". Many passengers arriving at both stations had to change onto buses, trams, and suburban rail services to complete their journeys further north.[39][note 4] The GNR attempted to address this issue by considering electrification several times, but to no avail due to shortage of funds. Meanwhile, the LER proposed an extension in 1920 but was overruled by the GNR, which was widely regarded as "unreasonable". In 1923, a petition by the Middlesex Federation of Ratepayers to repeal the 1902 parliamentary act emerged. It was reported that a "fierce exchange of arguments" occurred during a parliament session in March 1924 to request this change.[40][38] Frank Pick, as the new assistant managing director of the Underground, distributed photographs of the congestion at Finsbury Park to the press. All of this pressure finally prompted the government to initiate "The North and North-East London Traffic Inquiry", with initial reports only recommending a one-station extension to Manor House. 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. With funds still being insufficient to electrify the railway, the LNER reluctantly agreed to the latter.[37] An extension was highly likely at this stage, based on a study in October 1925 by the London & Home Counties Traffic Advisory Committee.[41][42]

Pick, together with the Underground board, began working on the extension proposal. Much pressure was also received from a few districts such as Tottenham and Harringay, but it was decided that the optimal route would be the midpoint of the GNR and the Hertford Line.[note 5] This was backed by the committee, and parliamentary approval for the extension was obtained on 4 June 1930, under the London Electric Metropolitan District Central London and City and South London Railway Companies Act, 1930.[43][44][note 6] Funding was obtained from legislation under the Development (Loan Guarantees and Grants) Act instead of the Trade Facilities Act. 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. In November 1929, the projected terminus was shifted further north to Cockfosters to accommodate a larger depot. It was estimated that ridership on the extension, which would cost £4.4 million, would be 36 million passengers a year.[45] In addition to Enfield West, stations were designated at Southgate, Arnos Grove, Bounds Green, Wood Green, Turnpike Lane, and Manor House. Bounds Green station was almost cancelled in order to improve journey times.[49] A more expensive provision was rejected, which included construction of a third track between Finsbury Park and Wood Green and an additional station at St. Ann's Road.[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. Twenty-two tunnelling shields were used for the tunnels,[51] and tunnel diameters were slightly larger than the old section, at 12 ft (3.7 m). Sharp curves were also avoided to promote higher average speeds on the extension. Platforms 400 ft (120 m) long were originally planned for each station to fit 8-car trains, but were cut short to 385 ft (117 m) when built. Some stations were also built with wider platform tunnels to cater to expected high patronage. To connect with buses and trams, interchange stations were provided with exits which led passengers directly to the bus terminal or tram stop from the subsurface ticket hall. The exits were purposed to improve connections which avoided chaotic passenger flow such as at Finsbury Park. Wood Green was an exception due to engineering difficulties, with the ticket hall at street level instead. Ventilation shafts were provided at Finsbury Park Tennis Courts, Colina Road and Nightingale Road, supplementing the existing fans within the stations. Provisions for future branch lines to Enfield and Tottenham were made at Wood Green and Manor House respectively, both to have reversing sidings. This had since changed, with only a reversing siding built at Wood Green and no provision for the branch line. Arnos Grove was built to have four platforms facing three tracks for trains to reverse regularly, with seven stabling sidings instead of one reversing siding and two platforms.[52][53][54]

Most of the tunnelling works were completed by October 1931, with the Wood Green and Bounds Green station tunnels done by the end of the year.[44] The first phase of the extension to Arnos Grove opened on 19 September 1932, without ceremony. The line was further extended to Enfield West on 13 March 1933 and finally to Cockfosters on 31 July 1933, again without ceremonies.[42][55] The total length of the extension was 7.6 mi (12.3 km).[3] Free tickets were distributed to residents on the first days of service on each extension. Initial ridership was 25 million at the end of 1933, which sharply increased to 70 million by 1951.[56] Despite there being no official opening ceremonies, the Prince of Wales visited the extension on 14 February 1933.[57][note 8]

The Hounslow West (then Hounslow Barracks) extension of the Piccadilly line, together with the Uxbridge extension, aimed to improve services on the District line which at the time were serving both branches from Acton Town (then Mill Hill Park).[59][note 9] The Uxbridge extension followed along existing routes on the DR and Met. The DR opened a spur from Ealing Common to South Harrow in June 1903. The Met opened its extension to Uxbridge in July 1904.[61] Through trains of the DR were eventually extended to Uxbridge on 1 March 1910, henceforth sharing tracks with the Met between Rayners Lane and Uxbridge.[62][note 10]

The viaduct from Studland Road (now Studland Street) Junction west of Hammersmith to Turnham Green was quadrupled on 3 November 1911. The London and South Western Railway (L&SWR) used the northern pair of tracks while the District Railway used the southern pair.[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. It would connect with the L&SWR tracks at Turnham Green.[66] It was approved as the London Electric Railway Act, 1913 on 15 August 1913,[67] but World War I resulted in no works done on the extension.[68] A Parliamentary report of 1919 recommended through running to Richmond and Ealing.[69] The Richmond extension plan was revived in 1922 by Lord Ashfield, the Underground's chairman. It was decided that the Piccadilly line extension was favourable over the CLR's as it was cheaper and had more capacity available.[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. Demand was also low on the South Harrow branch because of infrequent services and competition among other rail lines in the vicinity of each station. This prompted the Piccadilly line extension to be an express service between Hammersmith and Acton Town, with the future Heathrow Airport extension safeguarded in 40 years' time.[73] The Piccadilly line would run on the inner pair of tracks, and the District line on the outer.[69] Permission was granted to quadruple tracks to Acton Town in 1926 in conjunction with permit renewal for the extension. The Richmond extension never happened, but provisions allocated would allow this option to be revisited later.[74] Extensions would instead be to Hounslow Barracks and South Harrow, taking over DR services to the latter, with an estimated cost of £2.3 million.[75][note 12] In 1930, unsuccessful negotiations were made between LER and the Met to extend Piccadilly line trains to Rayners Lane for passengers to change trains.[77]

In 1929, quadrupling was to extend to Northfields for express trains to terminate there. This work was completed on 18 December 1932. Overall works for the extension began in 1931, approximately a year after permission was granted and funded under the Development (Loan Guarantees and Grants) Act of 1929. The Studland Road Junction area was partially rebuilt, with some of the old viaducts retained to date. The junctions diverging to Richmond were reconfigured at Turnham Green. Reversing facilities were initially designated at the latter, but these were not built.[78] Trial runs of Piccadilly line trains began on 27 June 1932. On 4 July 1932, services were extended to South Harrow, which replaced DR services. Northfields services were introduced on 9 January 1933, and on 13 March, were extended to Hounslow West. On 1 July 1933, the London Passenger Transport Board (LPTB) was formed, which included the Met, the DR and LER.[24] The board decided that there was sufficient demand to run through trains to Uxbridge due to rapidly developing suburbs along the line. The extension of Piccadilly line trains to Uxbridge began on 23 October 1933, but with many trains still reversing at South Harrow. By then, most Piccadilly line trains continued beyond Hammersmith, and District line trains to Hounslow were reduced to off-peak shuttles to Acton Town. An enhanced off-peak Piccadilly line service was introduced on 29 April 1935, cutting off-peak District line services down to the Acton Town–South Acton shuttle.[79] South Harrow short trips proved to be an inconvenience. The solution was to move reversing facilities to Rayners Lane. A new reversing siding was built there in 1935, which allowed some peak hour trains to terminate beginning in May 1936. Regular reversals were fully implemented in October 1943.[80] Peak-hour District line trains to Hounslow were fully withdrawn on 9 October 1964.[81]

In conjunction with the new extensions, several stations were considered for closure to increase overall line speeds. Down Street closed on 21 May 1932, Brompton Road on 29 July 1934, and York Road on 17 September 1932.[55][82] All three stations were lightly used, with Down Street and Brompton Road replaced by relocated entrances at Hyde Park Corner and Knightsbridge respectively. Knightsbridge's new below-ground ticket hall required stairwells from the entrance, one of which took over part of the Barclays Bank branch there. Both of the latter two stations retained their existing platforms, but the access from the surface was reconstructed with their entrances closer to the closed stations. These new entrances were provided with escalators, which replaced the lifts, improving passenger circulation. The Aldwych branch was deemed unprofitable, and in 1929 an extension to Waterloo was approved, costed at £750,000. No progress was made on the extension. Dover Street (now Green Park), Leicester Square and Holborn stations received new sets of escalators, with the lattermost having four in a single shaft. These were completed in the early 1930s.[83][84][85] As part of the 1935–40 New Works Programme, Earl's Court was largely reconstructed at street level. 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.[86][note 13] As a result, Russell Square station retained its lifts.[88]

To prepare for World War II, several stations had blast walls added. Others, such as Green Park, Knightsbridge and King's Cross St Pancras, had floodgates installed. The line was also involved in the evacuation of 200,000 children, by transporting them towards both ends of the line, then transferring them to mainline trains to continue their journeys to different country distribution hubs.[89] Some underground stations were fitted with bunk beds, toilets and first aid facilities, and sewerage. The disused Down Street was converted to an underground bunker for government use.[90] Other stations such as Holborn and Earl's Court also had essential wartime uses. The former had the Aldwych branch platforms as the wartime engineering quarters whilst the branch service was temporarily closed.[91] The latter produced Torpedo Data Computers at the transfer concourse between the District and Piccadilly lines.[89] Aldwych station was used as storage for British Museum exhibits.[91] On 13 October 1940, a bomb explosion caused the westbound platform tunnel at Bounds Green station to collapse, killing nineteen shelterers. Train services were suspended for two months.[92][93]

Reconfiguration of tunnels and platforms at Finsbury Park

In preparation for the Victoria line, cross-platform interchange was to be provided at a few stations, which included Finsbury Park on the Piccadilly line.[94] This meant that the Piccadilly line had to be realigned there, and the Northern City line platforms, being parallel to the existing Piccadilly line platforms, were to be transferred to the pair of lines. The Northern City line would be redirected to the surface platforms. The westbound Piccadilly line track would be rerouted onto one of these platforms, with the southbound Victoria line using the other. The northbound Victoria line would reuse the old westbound Piccadilly line platform and a part of the old tunnels, with the Piccadilly line diversion tunnels spanning 3,150 ft (960 m).[95]

Construction of the diversion began in October 1964, with the Northern City line having a temporary closure.[96] At the northern junction, step plate junctions were built to divert the existing line when the new tunnels were complete. They were fitted into the original Northern City line tunnels which had a greater tunnel diameter until two running tunnels were able to merge. The old and unused running tunnel was disconnected and blocked off when the junction tunnel was near its completion. Alteration of temporary points junctions and shifting of signals completed the diversion tunnels. In the south, the Piccadilly would be diverted to descend sharply under the northbound Victoria line tunnel, and then ascending to the original level which had a difference of 5 ft (1.5 m) approximately 200 ft (61 m) north of Arsenal station. The old westbound tunnel had to be supported on a trestle for works to be done. The trestle and old tracks were entirely removed once the diversion was ready for switchover. New tracks were laid at a rapid rate; it was done in about thirteen hours on 3 October 1965.[95] Both lines were connected via junctions south of Finsbury Park for stock movement and engineering trains. It was intended for Green Park to have cross-platform interchange, but was deemed impossible due to the lines crossing at right angles. The Victoria line opened on 1 September 1968 from Walthamstow Central to Highbury & Islington via Finsbury Park, and on 7 March 1969 to Warren Street via King's Cross St Pancras,[97] providing relief to the Piccadilly line.[98]

Inside a Piccadilly line carriage

To cater to the rapid growth of road traffic to Heathrow Airport, several rail lines were considered to serve the airport. An average increment of 1 million passengers a year between 1953 and 1973, and rising issues with airline coach services from major terminals due to location, traffic congestion, larger aircraft capacity and increasing leisure travel further increased the need for public transport connections. Other than the Piccadilly line extension from Hounslow West,[note 14] a Southern Railway spur (section now transferred to part of South Western Railway) from Feltham was also contemplated. These schemes were brought into parliamentary discussion in November 1966, and were approved with the Royal Assent as the London Transport Act 1967 and British Railways Act 1967 respectively on 27 July 1967.[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". Platforms at Hounslow West had to be relocated below-ground to the north of the existing for the new track alignment. The 1931 ticket hall was retained, with connections to the new platforms. A cut-and-cover excavation method was used between Hounslow West and Hatton Cross, a new station on the extension. This 2-mile section had a shallow trench dug, with the tunnel walls supported by intersecting concrete piles. The line had to cross the River Crane just east of Hatton Cross, therefore it emerges briefly on a bridge, with the two portals having concrete retaining walls. Deep tube tunnels were bored from Hatton Cross to Heathrow Central (now Heathrow Terminals 2 & 3). On 19 July 1975, the line was extended to Hatton Cross.[102][note 15] The Heathrow Central extension was inaugurated by the Queen around noon on 16 December 1977, with revenue services commencing at 3pm.[104]

In the 1970s, planning was already underway for a fourth terminal for the airport, and its location was to be to the southeast of the existing terminals. As the Piccadilly line's route to the existing terminals was out of place, a loop track was adopted as the best method to serve the new terminal. The westbound track between Hatton Cross and Heathrow Central would be retained for emergency services. Permissions for constructing for the loop was approved and received Royal Assent under the London Transport Act 1981 on 30 October 1981.[105] On 19 July 1982, the original location of the station and track alignment were altered to compensate for the British Airport Authority (BAA) to finish the fourth terminal building which was behind schedule. Construction of the 2.5 mi (4.0 km) extension began on 9 February 1983, with an estimated cost of £24.6 million. Tunnelling for the loop was completed in seventeen months. It was expected that the extension would open with the new terminal. However, the terminal opening was delayed, with the loop service completed and commissioned on 4 November 1985. The terminal and station were finally opened a few months later on 1 April 1986, by the Prince and Princess of Wales. Regular traffic began twelve days later with trains serving Terminal 4 via a one-way loop to Terminals 1,2,3.[106] The station only has a single platform, the only one with this configuration on the Piccadilly line.[54]

Terminal 5 required another extension, funded by BAA. However, its proposed alignment caused an issue: it was reported that London Underground was unhappy with the terminal's location on the site of the old Perry Oaks sludge works which was originally intended for Terminal 4.[9] It was now impossible for all three terminals to be served on the same route, and the final solution was to have twin tunnels serving Terminal 5 from Terminals 1,2,3. From 7 January 2005 until 17 September 2006, the loop via Terminal 4 was closed to allow this connection to be built. Terminals 1,2,3 became a temporary terminus; shuttle buses served Terminal 4 from the Hatton Cross bus station.[107] Part of the junction between the through and loop tracks needed to be rebuilt. The Terminal 5 project team shut down two aircraft stands from Terminal 3 so that an access shaft could be constructed. The new junction was then built into a concrete box which connected all the underground tunnels.[108] The station and terminal were opened on 27 March 2008, splitting westbound Piccadilly line services into two: one via the Terminal 4 loop, another direct to Terminal 5.[109][110]

Plans to extend the Aldwych branch south to Waterloo was revived several times during the station's life. The extension was considered in 1919 and 1948, but no progress towards constructing the link was made.[91]

Memorial plaque of the King's Cross fire erected at the station itself.

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. In March 1965, a British Rail and London Transport joint planning committee published "A Railway Plan for London", which proposed a new tube railway, the Fleet line (later renamed the Jubilee line), to join the Bakerloo line at Baker Street then run via Aldwych and into the City of London before heading into south-east London. An interchange was proposed at Aldwych and a second recommendation of the report was the revival of the link from Aldwych to Waterloo.[111][112] London Transport had already sought parliamentary approval to construct tunnels from Aldwych to Waterloo in November 1964,[113] and in August 1965, parliamentary powers were granted. Detailed planning took place, although public spending cuts led to postponement of the scheme in 1967 before tenders were invited.[114]

With the Aldwych branch receiving no extensions, it remained a lightly used shuttle service from Holborn. The branch was considered for closure many times, but it survived.[115] Saturday services were fully withdrawn on 5 August 1962.[116] Maintenance costs of replacing the aged lifts were high at over £3 million, which failed to meet safety standards at the time. In August 1993, a public inquiry was held for closure of the short branch line. On 30 September 1994, the branch was closed to traffic.[note 16] The disused station is now used for commercial filming and as a training facility.[117]

On 18 November 1987, the large King's Cross fire broke out, the incident being near the Northern/Piccadilly line escalators which killed 31 people.[118][119] As a result, wooden escalators were replaced at all Underground stations.[120][121][note 17] The Piccadilly line platforms remained open, but with the escalators to the ticket hall closed for repairs. Access was temporarily via the Victoria line or Midland City platforms. New escalators were fully installed on 27 February 1989.[125]

On 7 July 2005, a Piccadilly line train was attacked by suicide bomber Germaine Lindsay in the day's London bombings.[126] The blast occurred at 08:50 BST while the train was between King's Cross St Pancras and Russell Square. It was part of a co-ordinated Islamist terrorist attack on London's transport network, and was synchronised with three other attacks: two on the Circle line and one on a bus at Tavistock Square. The Piccadilly line bomb led to the largest number of fatalities, with 26 people reported killed. Owing to it being a deep-level line, evacuation of station users and access for the emergency services proved difficult.[127] Shuttle services were introduced between Hyde Park Corner and the Heathrow loop, between Acton Town and Rayners Lane, and between Arnos Grove and Cockfosters. Full service was restored on 4 August, four weeks after the bombing.[118][110]

On 15 December 2006, a 100-year celebration of the Piccadilly line was launched. A birthday card was revealed by Tim O'Toole, then London Underground Managing Director at Leicester Square station.[128]

Lots Road Power Station, disused and planned to be redeveloped

The line from Cockfosters to Heathrow and South Harrow is controlled from a control centre at South Kensington,[156] which replaced the old Earl's Court control centre, which was shared with the District line.[157] Rayners Lane signal cabin is responsible for signalling the Piccadilly line from just northwest of South Harrow to Uxbridge, as well as the Metropolitan line joining at Rayners Lane.[158] The signalling system is expected to be upgraded in line with the Deep Tube upgrade, which will increase line frequencies up to 33 tph.[156] Trains may also be able to make a permanent additional stop at Turnham Green following this upgrade.[159]

When the line was opened from Finsbury Park to Hammersmith, its signalling system was identical to the Bakerloo and District lines. Small cabins at each location of crossover installed controlled the signals there. Single lamps displayed track clearance in the form of green or red, with variations of yellow at difficult-to-spot locations. This equipment was supplied by Westinghouse and operated using compressed air. The exception was between West Kensington and Hammersmith, where it was controlled by District Railway signal boxes, and had semaphore signals instead.[160][161] The Piccadilly line extensions resulted in resignalling on tracks west of Barons Court. Signal cabins were adjusted and new ones were added at Hammersmith, Acton Town and Northfields. A mixture of semaphore and colour-light signals were used on the four-track section. Signalling was redone on the new Uxbridge branch.[162][note 20]

Speed control was introduced at several stations to enhance the signalling system after World War II. This enabled a train to proceed slowly into an occupied platform without stopping in front of another before it departed, thus improving headways. The last semaphore signal, at Ealing Common, was replaced in November 1953.[163] A control room was built at Earl's Court to centralise supervision of most of the line signalling in the 1960s, while Rayners Lane signal cabin was, and still is, the main control centre of the Rayners Lane to Uxbridge portion;[164] shared with the Metropolitan line.[54] 1930s automatic signalling equipment was updated in the 1970s and 1980s.[165]

The UERL built a large power station that would be capable of providing power for the District line and the underground lines planned. Work began in 1902 at Lots Road, by Chelsea Creek, and in February 1905 Lots Road Power Station began generating electricity at 11 kV 33+1⁄3 Hz, conveyed by high-voltage cables to substations that converted this to approximately 550 V DC.[166] On the Piccadilly line, electricity was transmitted via underground ducts to Earl's Court, which was then distributed to different substations.[160][note 21] Power supply for the Cockfosters extension was initially generated by the North Metropolitan Electric Power Supply Company at Wood Green.[167] It was later supplied by Lots Road station. The National Grid network took over the supply on the Ravenscourt Park to Uxbridge and Northfields section. Lots Road was permanently closed on 31 October 2003, being also replaced by power supply from the National Grid Network. Emergency lighting, powered by batteries, is available at every station, with emergency supply from a support power station in Greenwich.[165]

Northfields Depot, with an S stock train.

The line has two depots, at Cockfosters^{map 54} and Northfields.^{map 55} The former site is near Trent Park, and was the preferred site over Oakwood, which was smaller,[45] and its only access point was south of the station. Subsequently, Oakwood was built with an island platform, with its intended three-track terminus layout moved to Cockfosters.[168] Light maintenance and cleaning of trains is done here, with the latter site, being the main depot, having train maintenance facilities.[165] Northfields depot is also double-ended, with access from just west of Boston Manor station[169] and Northfields station. For it to be built, the latter station had to be rebuilt nearer to South Ealing station.[150] The depot was opened earlier than the Hounslow extension, on 4 July 1932, and fully electrified two months later.[162][54][note 22]

Single sidings are placed at Rayners Lane, Oakwood, Down Street (Hyde Park Corner), and Wood Green. Arnos Grove, Acton Town, South Harrow, Uxbridge, Hammersmith and Heathrow Terminal 5 have more than one siding for reversing trains or storing them.[54] Four crossovers were built for the initial line opening in 1906,[160] Hounslow Central on 3 March 1923,[7] and double crossovers at South Harrow were added in the 1930s.[162]

Crossovers with other lines are present on the Piccadilly line. A connecting tunnel from the northbound Piccadilly line at King's Cross St Pancras to the northbound Northern line Bank branch was constructed in 1927.[170][note 23] At Finsbury Park, a set of crossovers in the south were retained where trains can cross over onto the Victoria line.[98]

Lillie Bridge was the main depot when the Piccadilly line was initially opened. Trains entering service on the line had to reverse and enter the District line tracks first via West Kensington.[171][172] When the Piccadilly and District line tracks were realigned in the 1930s, access points into the depot had to be altered.[173][174] When the line was extended to Northfields and Cockfosters in 1933, all trains except seven[note 24] were stabled at the newer depot.[162] Once the Cockfosters depot was opened, Lillie Bridge was converted to a maintenance depot, where it housed only engineers' and materials trains.[175][172]

Most original deep-level stations were installed with lifts and stairs, with some descending directly down to platform level.[176] Many of these were given an overhaul in the 1930s with escalators replacing lifts for quicker passenger flow.[83][177] Underground stations on the Cockfosters extension were built with access mainly via escalators; each station shaft are able to fit three escalators, but some stations had two escalators with a stairwell by the middle. Escalators at Bounds Green, Wood Green and Manor House travel at 165 feet (50 m) per minute, which were then the quickest on the network.[31] All of the original lifts were either replaced by new equipment or were converted to escalators. Alperton was the only above ground station to have an escalator, which was transferred over from the Festival of Britain, but was decommissioned in 1988.[163] 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.[178][179][180][181]

On 11 October 1909, peak-hour services were altered to have trains skipping certain stops to improve journey times. Trains were marked with "Non Stop", which were deemed unpopular and ambiguous among passengers. Illuminated signs were added on platforms in 1932 to address this issue. Pairs of stations were the norm, such as Holloway Road and York Road, and Caledonian Road and Gillespie Road. After World War I, Covent Garden, Russell Square, South Kensington, Brompton Road and Gloucester Road were among the stations skipped. Boston Manor, South Ealing, North Ealing and Barons Court were included into these patterns by 1938. Skip-stop services were discontinued in June 1947.[186]

In 1930, during the planning of the northward and westward extensions, a 30tph peak-hour service was proposed between Wood Green and Turnham Green. This was preferred over an alternate skip-stop service through Bounds Green.[187][note 25] When the Piccadilly line was initially extended to Northfields in 1933, South Ealing was skipped. It was eventually served on 29 April 1935 by off-peak trains, and peak services in May 1942.[188][note 26]

When the Piccadilly line shared its storage of trains with the District line at Lillie Bridge Depot,[171] some trains started and ended their service at West Kensington; a few of these ran empty between West Kensington and Hammersmith. These services ceased on 27 October 1991.[163]

In the late 1990s, the Labour government initiated a public–private partnership (PPP) to reverse years of underinvestment in London Underground.[199] Tube Lines planned to order 93 new Piccadilly line trains, which would enter service by 2014.[200] In January 2007, Tube Lines started the process of ordering new Piccadilly line trains, by querying if train manufacturers would be interested in supplying them.[201] Contract award was anticipated for 2008, and trains would enter service on the Piccadilly line in 2014.[201] However, TfL bought out the Tube Lines consortium after cost overruns in 2010, formally ending the PPP.[202]

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.[203] A feasibility study into the new trains showed that new generation trains and re-signalling could increase capacity on the Piccadilly line by 60%, with 36 trains per hour.[203]

In June 2018, the Siemens Mobility Inspiro design was selected.[204] These trains will have an open gangway design, wider doorways, air conditioning and the ability to run automatically with a new signalling system.[205] However, TfL could only afford to order 94 trains at a cost of £1.5bn, and not the cost of resignalling and automating the line.[206] Half of the trains will be built in Goole, East Riding of Yorkshire.[207] Trains are expected to enter service in 2025.[208]

The line was previously worked by the 1906 gate stock, which were manufactured in France and Hungary.[176] The 52 ft (16 m) carriages were connected to form three, four or five-car trains, and were designed to have a maximum of six cars per train. Built with steel as its main material, the interiors were covered by fireproof mahogany veneer.[209] Six-car trains were implemented in 1917. Additional cars were ordered and brought into service between 1920 and 1921 to combat shortage of capacity.[210] In late 1920s, plans were developed to replace these outdated stocks. Metropolitan Carriage, Wagon & Finance Company (Metro-Cammell's predecessor) was to build standard tube stocks, which were delivered within 1928 and 1929. Another batch of these were constructed in Feltham by the Union Construction company. All the gate stocks were decommissioned in June 1929.[211] The new standard tube stocks have some wider doors on trailer cars and extra doors on the motor car. All doors are now air-operated.[212][note 29] In 1929, it was suggested that a new prototype of tube stock should include three double doors on each side. These seven-car trains, also standard tube stocks, had carriages at least one foot longer. They were made to be lighter, and had better interior lighting. These new trains were completed in two periods, 1931 and 1934, which costed £1.20 million in total.[214][215]

Due to ridership increasing by 61 per cent on the Uxbridge branch between 1931 and 1938, Piccadilly line trains were packed with passengers. New experimental trains were brought into service in 1936, which were formed of four or six cars. These trains had their control equipment (of seven different types from four different manufacturers) placed under the car floor, which enabled more space for passengers on both motor cars. While boarding times were decreased, some of the designs of control equipment were said to be less reliable than others. Nevertheless, they were a prototype for the design of the 1938 stock.[216][217] As part of the 1935–40 New Works Programme, the latter tube stock was introduced onto the Piccadilly line,[218] and were regarded as the most advanced tube stock at the time. Its interiors featured art deco lighting, depicted similar to a 1930s stylish restaurant.[219][note 30] As a safety measure for World War II, trains initially had their lights switched off at night. It was later amended to illuminate trains with dim blue lamps, and then, largely enclosed reading lamps.[221] Patronage on the line increased greatly after the war, which meant additional trains were needed. New 1938 stocks, together with the experimental trains were redone to form as a fleet of seven-car 1949 stock, in operation from 1952.[222][223][note 31]

New test trains were manufactured again in the 1950s, and three prototype trains, branded as the 1956 stock, were trialled on the Piccadilly line in 1957 and 1958. These new trains would replace most of the 1938 tube stocks. It was successful, and 76 seven-car trains were placed on order as the 1959 stock.[225] These were to be built by Metro-Cammell, with its first train entering service on 14 December 1959.[226][227][note 32] The standard stocks were withdrawn in 1964,[228] and the 1938 stocks were slowly taken out of service due to age. The 1959 stocks were gradually transferred to the Northern line between November 1975 and October 1979, being replaced by the 1973 stock trains; the last of the latter coming into service on 5 October 1979.[192]

A few 1938 trains were preserved, and in June 2011, one of these trains made a few shuttle trips on the line as part of an event in conjunction with Father's Day.[230]

The Aldwych branch

• Aldwych opened on 30 November 1907 as Strand. It was renamed Aldwych on 9 May 1915.[55] The possibility of extending the branch to Waterloo was discussed, but the scheme never proceeded.[251] Aldwych was closed on 30 September 1994. It is now regularly used by film makers.[252]
• Brompton Road opened 15 December 1906; closed 30 July 1934, between Knightsbridge and South Kensington.[55]
• Down Street opened 15 December 1906; closed 21 May 1932, between Green Park and Hyde Park Corner.[55]
• Osterley & Spring Grove first served on 13 March 1933;[79] closed 24 March 1934, between Boston Manor and Hounslow East. It was replaced by Osterley.[55]
• Park Royal & Twyford Abbey opened 23 June 1903; closed 5 July 1931. Although on the route of the current Piccadilly line, a short distance north of the present Park Royal station, it was never served by Piccadilly line trains. It was opened by the District Railway, and was closed and replaced by Park Royal[55] before the Piccadilly line started running trains to South Harrow in 1932.[79]
• York Road opened 15 December 1906;[17] closed 19 September 1932, between King's Cross St Pancras and Caledonian Road.[82] It was about 600 m north of King's Cross St Pancras.[253]