Mayor asked to reveal Piccadilly line train issue

The new trains were planned to enter service this year, but issues have pushed this back to 2026

London's mayor Sir Sadiq Khan is facing calls to reveal the issue that has delayed the rollout of new Piccadilly line trains.

Conservative member of the London Assembly, Keith Prince, said there needed to be transparency about the "potentially safety-critical" issue that has delayed the trains by about a year, according to the Local Democracy Reporting Service.

Sir Sadiq told the London Assembly last month that "the process is taking longer than expected due to challenges with the design, build and commissioning".

The 94-strong fleet was originally scheduled to enter service this year, but this has been pushed back to the second half of 2026.

The new models are planned to replace the current fleet from the 1970s.

They are air-conditioned, with walk-through carriages, wider doorways and on-board CCTV.

The trains arrived in the UK for testing in late 2024 to enter service later this year, but this was delayed after technical issues were identified on a prototype.

This meant the trains could be damaged unless they were modified.

Transport for London (TfL) began testing the trains on 2 and 3 August.

They were run between Northfields and Hyde Park Corner during a scheduled closure of the Piccadilly Line.

The mayor celebrated this news as "exciting" in an X post on 7 August, which showed one of the trains arriving at South Kensington station.

Prince said: "Instead of being excited, he should be concerned and pressing TfL to get a move on with this.

"The last we heard was that there is a fundamental issue which will require a significant change to the trains, although no-one is coming clean about what the issue is."

TfL said there had been engineering and design challenges due to the depth and length of the Piccadilly line

The Local Democracy Reporting Service asked the mayor's office what the exact issue was with the trains, but had not yet received a response.

TfL commissioner Andy Lord told the London Assembly last month that the new model was "extremely complex and we need to make sure it is introduced both safely and reliably".

He said there had been "some issues" between the manufacturer and TfL, with further modifications needed.

TfL previously said there had been engineering and design challenges due to the Piccadilly line being one of the deepest and longest Tube lines.

Siemens Mobility and TfL both said it would take up to a year to test the trains, partly due to the tunnel infrastructure, which is almost 120 years old.