A whole new fleet of air-conditioned trains where you can walk through from carriage to carriage is planned for the Piccadilly Line next year.

Eight-out-of-ten trains on the service are being replaced, with a new assembly plant being built in Yorkshire and due to open in the spring.

The company making the trains, Siemens Mobility, is in the final stages of constructing the new factory where most of the new trains are to be put together as part of a £200 million Transport for London investment.

“We are assembling trains here in Britain for the first time,” Siemens Mobility’s joint chief executive Sambit Banerjee said.

“This is a milestone for the industry to build 80 per cent of Piccadilly line trains in Yorkshire, which will transform travel for commuters in London.

“The next generation of walk-through trains are being assembled by the next generation of engineers.”

The new factory is also expected to replace the fleet of Bakerloo line trains, which is more than 50 years old and the oldest in passenger service in the UK.

The first new train for the Piccadilly line, however, has already been made at the company’s factory in Vienna and has been undergoing testing by engineers in Germany, which is due to arrive in London this summer.

But all future orders by TfL, including the new trains for the Bakerloo, are being assembled in Yorkshire once further funding is secured from the Government to renew London Underground’s ageing rolling stock.

TfL is to carry out further testing and integrating the new Piccadilly line trains before they go into public service in 2025, running straight through from Cockfosters to Heathrow Airport and Uxbridge.

The longer trains with end-to-end walk-through carriages, similar to those introduced on the Metropolitan, District, Circle and Hammersmith lines from 2014, have around 10 per cent more passenger capacity than the older ones they’re replacing.

They are also fitted with digital screens with ‘real time’ passenger information and have CCTV in the carriages.

The design is lighter than existing trains, with fewer rail bogies, which make them more energy-efficient as well as giving a smoother ride.