Transport for London has not revealed what the problem is despite repeated questions, reports Kumail Jaffer, Local Democracy Reporter

A new Piccadilly Line train running on a test track (credit Seimens)

A London Assembly member has called on Sir Sadiq Khan to reveal exactly what issue was identified on the new Piccadilly Line trains that has led to their launch being delayed by a year.

The new trains have finally been tested in tunnels deep beneath London ahead of their rollout for public use, a milestone that’s been hailed by the mayor as “exciting”.

But Conservative London Assembly Member Keith Prince has called for transparency around the nature of the issue which was deemed potentially safety-critical.

He told the Local Democracy Reporting Service (LDRS): “It’s interesting that the mayor is excited about the Piccadilly Line trains that, so far, are a year behind schedule – as are most of the projects he oversees.

“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 – so much for a transparent mayor.

“Instead of being excited, he should be concerned and pressing TfL to get a move on with this.”

Transport for London (TfL) released a video of the new, state-of-the-art train arriving at South Kensington Station while the Piccadilly Line was closed last weekend.

The air-conditioned tube train, which also boasts walk-through carriages, wider doorways and on-board CCTV, was tested between Northfields and Hyde Park Corner.

TfL had originally hoped that the 94-strong fleet of new models, which would replace the current fleet from the 1970s, would start entering service later this year after arriving in the UK in late 2024 for testing.

But technical issues identified on a prototype meant that trains could be damaged unless they were modified. They are now scheduled in for passenger use in the second half of 2026.

Mayor Khan said yesterday (Thursday 7th): “Exciting changes are coming to the Piccadilly Line. New state-of-the-art-trains are now being tested by TfL.”

TfL’s chief capital officer Stuart Harvey claimed the trains will “transform journeys for millions of customers”.

He said: “We’ve now started a rigorous programme of testing the new Piccadilly Line trains with London Underground’s infrastructure and systems. The first test runs have taken place on the Piccadilly Line between Northfields and Hyde Park Corner.

“The new trains, which will transform journeys for millions of customers on one of the underground’s busiest lines, are planned to start operating in the second half of 2026.”

TfL has previously said that some of the engineering and design challenges stem from the fact that the Piccadilly Line is one of the longest and deepest tube lines.

In a written answer to the London Assembly last month, Khan said: “Introducing a fleet of 94 new state-of-the-art trains to one of the deepest and longest lines on our network is a hugely complex task.

“This process is taking longer than expected due to some challenges associated with the design, build and commissioning of this innovative new train, while also ensuring it is fully integrated with TfL infrastructure.”

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

He said “some issues” existed between both tube manufacturer Siemens Mobility and TfL and that further “modifications” were needed.

Khan’s office was asked by the LDRS what the exact issue is with the trains but has not yet received a response.

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