When do the new Piccadilly line trains start running?

The new Piccadilly line trains are expected to carry their first passengers between December 2026 and June 2027, Transport for London's current published window as of 31 August 2026 (Made by TfL). That date has already slipped twice from an original 2024 target, so treat it as a range, not a fixed day.

• Status : date-sensitive, testing in progress on the Piccadilly line
• Official target : first passenger service between December 2026 and June 2027
• Last updated : 31 August 2026
• Next expected milestone : daylight passenger-carrying trials on the Piccadilly line, ahead of the first scheduled service

This page tracks the Piccadilly line new trains rollout as an ongoing story: the rolling stock itself, the civil and power works needed to run it, the delays so far, and the independent scrutiny of the programme's cost and schedule. If you enjoy testing your own knowledge of the network in the meantime, you can play today's London Underground map game or browse more London stories on the blog.

Why is this changing?

The trains being retired are the London Underground 1973 Tube Stock, the same fleet Rail Engineer's technical coverage of the replacement programme refers to throughout as "1973 tube stock" (Rail Engineer). Despite the name, that designation is the year the stock was ordered and built (1973 to 1977 by Metro-Cammell), not the year it started carrying passengers: it entered service in 1975 with the Piccadilly line's extension to Hatton Cross (Made by TfL). So the fleet is usually described as "trains dating from 1975" in press coverage even though the correct rolling-stock designation is 1973 Stock. It is the second-oldest working fleet on the Underground after the Bakerloo line's 1972 Stock, which TfL's own chief capital officer has said is "even older" and next in line for replacement (Rail Engineer).

The replacement is the London Underground 2024 Tube Stock, built by Siemens Mobility on its Inspiro platform and marketed as Inspiro London. Siemens Mobility was named preferred bidder on 15 June 2018, and London Underground formally signed the contract on 20 November 2018, worth approximately £1.5 billion for the initial order of 94 trainsets plus a 40-year fleet services agreement (Railway Gazette International). Siemens Mobility's rolling stock director called air conditioning "a real challenge in hot tunnels" even at the time of signing, underlining how central the cooling problem was to the design brief from day one. TfL ordered 94 nine-car trains for the Piccadilly line, with a contract option for up to 250 trains across four deep-tube lines (Piccadilly, Bakerloo, Central and Waterloo & City) as part of what TfL originally called the Deep Tube Upgrade Programme (DTUP) (TfL Programmes and Investment Committee paper, 2018).

The most technically demanding part of the design is air conditioning inside a deep-level tube tunnel, something no Piccadilly, Bakerloo, Central or Waterloo & City train has ever had. Deep tube tunnels are narrow (the "kinematic envelope" leaves little room for rooftop or underfloor plant) and have very few vent shafts to the surface, so air conditioning cannot simply dump heat outside the train the way a surface railway's units do; the heat has to go somewhere in a tunnel that is already warming up (CIBSE Journal). Peer-reviewed modelling of London's tunnels attributes most of that background heat to train braking and motor and auxiliary losses rather than to passengers themselves, and shows the tunnel walls absorbing the majority of it over time (Sadokierski and Thiffeault, "Heat Transfer in Underground Rail Tunnels"). The 2024 Stock's answer is a lighter, more efficient train (Siemens cites a 20% cut in energy consumption against the 1973 Stock) so that less heat is generated in the first place, plus a compact underfloor air-conditioning package using ten bogies across the nine-car train rather than the fourteen a conventional design would need (Rail Engineer).

The other design driver is capacity. The 2024 Stock uses a walk-through, articulated body (open gangways between cars instead of separate carriages), which Siemens says lets each 113-metre train carry around 1,042 people, roughly 10% more than a 106-metre 1973 Stock train (Siemens Mobility press release). Wider double doorways are meant to cut the time trains spend at platforms, which matters directly for the planned frequency increase described below.

What's changing on the Piccadilly line, and when

The upgrade covers the whole line end to end (Cockfosters to Heathrow and Uxbridge), not a subset of stations, because every platform and depot the new, longer trains touch needs some work.

• Testing began at Siemens' 44-hectare Wegberg-Wildenrath Test and Validation Centre in Germany, where the first complete train arrived from the Vienna factory in August 2023 for type testing and climate-chamber trials across a range from -15°C to 40°C (Rail Engineer).
• The first train reached London in October 2024 for driver and maintenance-staff training and on-network testing (Made by TfL).
• By April 2025, a 2024 Stock train had made its first main-line test run between Ruislip and Rayners Lane, and by mid-2025 several units were running engineering-hours test moves between Northfields and Oakwood (From the Murky Depths).
• TfL has not published a confirmed date for a Rickmansworth-branch test programme; treat any specific branch beyond Ruislip-Rayners Lane and Northfields-Oakwood as unconfirmed until TfL says otherwise.

What riders will notice

Fares and Oyster/contactless payment do not change with this rollout: it is a rolling-stock and infrastructure project, not a ticketing one, so pay-as-you-go, Travelcards and daily/weekly caps keep working exactly as they do today on any Piccadilly line train.

• Comfort: air conditioning on a deep-level line for the first time, plus walk-through carriages so passengers can move along the train rather than being confined to one car.
• Doors and dwell time: wider double doorways at every set of doors, intended to speed up boarding and alighting, especially at busy interchanges and at Heathrow's terminal stations.
• Accessibility: step-free improvements at some platforms, on-board CCTV, and design input from TfL's Independent Disability Advisory Group on layout and signage (Siemens Mobility press release).
• Frequency: once the fleet is established, peak-time trains are planned to run every 135 seconds (27 an hour) instead of every 150 seconds (24 an hour), a 23% capacity increase (CiTTi Magazine).
• Disruption while it's built: expect periodic weekend or short full closures on sections of the line for depot, power and platform works during the rollout window, as has already happened around Cockfosters and Northfields.
• For tourists using Heathrow: the Piccadilly line remains the direct rail link between all four Heathrow terminals and central London, and it carries more luggage than any other Underground line, which is why TfL surveyed luggage patterns to size the new trains' open gangway space and door widths (London Assembly Mayor's Question answer on Piccadilly line luggage). Nothing about the fare, the route or the roughly 50-minute journey to central London changes when the new trains arrive; the difference is more space to stand a case next to you and air conditioning on a trip that can otherwise get very warm.

Milestone and delay log

1. 15 June 2018: Siemens Mobility named preferred bidder for the new deep-tube fleet.
2. 20 November 2018: contract formally signed, initial order worth approximately £1.5 billion for 94 Piccadilly line trains (Railway Gazette International).
3. 2022 to 2023: first car bodies and complete trains built in Vienna; first complete train arrives at Wegberg-Wildenrath on 1 August 2023 for type testing (Railway Gazette International).
4. October 2024: first train delivered to London for on-network testing and staff training.
5. Original target missed: entry into passenger service, first planned for 2024/2025, was pushed back to "late 2025" and then to "the second half of 2026" as testing and infrastructure integration took longer than planned.
6. April 2025: first main-line test run, Ruislip to Rayners Lane.
7. Mid-2025: several 2024 Stock units running repeated engineering-hours test moves between Northfields and Oakwood (Modern Railways).
8. February 2026: TfL confirmed a second slip, to a December 2026 to June 2027 window, and disclosed the programme's cost had risen by £409 million to about £3.4 billion; TfL commissioner Andy Lord told the London Assembly the new train was "extremely complex" and that some issues existed between Siemens Mobility and TfL over integrating the trains with decades-old tunnel cabling and signalling equipment (London Assembly coverage).
9. As of 31 August 2026: no further slip has been announced beyond the December 2026 to June 2027 window; the frequency increase to 27 trains per hour is targeted for around mid-2027, after the fleet is established.

Independent risk view

No National Audit Office or equivalent value-for-money report focused specifically on the Piccadilly line trains or the Deep Tube Upgrade Programme was found during this research; if you know of one, it is not reflected here yet, and readers should not assume the absence means the programme has been independently cleared. The closest oversight to date is scrutiny from the London Assembly and TfL's own committee papers.

That scrutiny points to real risk in how far this rollout will reach beyond the Piccadilly line itself. The Piccadilly fleet order was placed with the explicit expectation that Siemens would later supply similar trains for the Bakerloo, Central and Waterloo & City lines under the wider Deep Tube Upgrade Programme (Railway Gazette International), but as of August 2026 only the Bakerloo follow-on has moved forward, and even that is not yet committed. TfL has appointed contractors to expand the Bakerloo's Stonebridge Park depot and is seeking a compulsory purchase order for adjoining land, subject to a board decision on whether to actually place the order (ianVisits). On current planning, TfL is aiming to introduce the new Bakerloo fleet only in the early 2030s, with one August 2026 report putting infrastructure works finishing in December 2034 and new trains on track by 2035 (Time Out London). The Central and Waterloo & City lines have no comparable public timeline at all. In effect, the programme that was meant to renew four lines at once has become a sequence of single-line decisions, each contingent on funding and land that TfL does not yet have secured.

Separately, London Assembly members have pressed TfL on transparency around the technical cause of the 2026 slip, since it was described internally as potentially safety-related before TfL characterised it as an integration and commissioning issue rather than a safety defect. TfL's own reporting confirms the Piccadilly fleet itself was still mid-testing as recently as May 2026, with only four trains running engineering-hours trials and daylight testing still "in the coming months" at that point (Modern Railways). The pattern across this programme, two schedule slips and a roughly £2 billion rise from the original contract value to the current £3.4 billion programme cost, is consistent with the kind of integration risk seen on other TfL signalling and rolling-stock projects, such as the Four Lines Modernisation programme's well-documented cost and schedule overruns. Riders and reporters should treat the December 2026 to June 2027 window as TfL's current best estimate, not a guarantee.

Sources

• Made by TfL, "Inside the Piccadilly line's major upgrade" (29 July 2026): TfL's own blog with the current December 2026 to June 2027 window and depot/power scope.
• TfL, Piccadilly line upgrade project page: official project page, cost, fleet and frequency figures.
• TfL Programmes and Investment Committee, Deep Tube Upgrade Programme update (16 May 2018): the original board paper defining DTUP's four-line scope.
• Siemens Mobility press release, detailed design of the Piccadilly line trains: manufacturer's own figures on capacity, energy use and accessibility design.
• Railway Gazette International, "Siemens signs £1.5bn London Underground train contract" (20 November 2018): trade press, contract value and award dates, and Siemens' own "real challenge in hot tunnels" quote.
• Railway Gazette International, "First of London Underground's new Piccadilly Line trains ready for testing" (1 August 2023): trade press, Wegberg-Wildenrath arrival date and test scope.
• Rail Engineer, "The new train on the Piccadilly line will arrive in 2025": technical trade press, the 1973 tube stock designation, bogie/underfloor air-conditioning detail, and TfL's own withdrawal-timeline comments.
• CIBSE Journal, "Tunnel vision: the challenge of cooling the London Underground": technical trade publication of the Chartered Institution of Building Services Engineers on deep-tube heat rejection.
• Sadokierski and Thiffeault, "Heat Transfer in Underground Rail Tunnels" (2007): peer-reviewed-style academic modelling paper on tunnel heat sources and soil heat storage.
• CiTTi Magazine, "Details of £2.9bn Piccadilly line upgrade revealed": trade press, Balfour Beatty power contract and depot detail.
• Modern Railways, "2024 Stock testing gathers pace on Piccadilly Line" (21 May 2026): trade press, on-network testing progress between Northfields and Oakwood and the still-provisional December entry-into-service target.
• From the Murky Depths, "New Piccadilly line tube train has first main line test": enthusiast trade coverage of the first Ruislip to Rayners Lane test run.
• Time Out London, coverage of the February 2026 delay announcement: press coverage of TfL's £409 million cost rise disclosure and Andy Lord's comments to the London Assembly.
• ianVisits, "Bakerloo line train upgrade moves forward as TfL seeks land for larger depots" (1 July 2026): trade press on how far the wider Deep Tube Upgrade Programme has actually progressed beyond the Piccadilly line.
• Time Out London, "The Bakerloo line's ancient trains are finally being replaced" (5 August 2026): press coverage of the Bakerloo line's separate, later timeline.
• London Assembly, Mayor's Question answer on Piccadilly line luggage: official record of TfL's luggage-pattern survey informing the new trains' layout.