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Air conditioning on London's Tube trains can actually make carriages hotter than they would be without it, according to engineers.

Paul Priestman, chairman of design firm PriestmanGoode, said cooling carriages on certain lines could have the opposite effect because of the heat generated by the process.

His company is currently working with Transport for London on designs for new trains set to launch on the Piccadilly, Bakerloo and Central lines from 2025.

He said the problem affected services on deep-level routes which the stock is being designed for. Some trains on "sub-surface" lines such as the Metropolitan and Circle are already effectively air conditioned but carriages on most other lines are not installed with the technology.

But Mr Priestman said engineers faced different challenges with lines such as the Piccadilly and Central, where the tunnels and stations are much deeper.

He said: “One of the problems is the doors are actually open longer than they’re shut…

“So what happens is you could cool the Tube, the doors open and all the cool air spills out on to the platform.

“But the trouble is the heat given off by the train to cool the inside of the train is then given into the platform.”

He told the BBC’s Today programme: “Basically what happens is you just heat up the system more and more and the old Tube stations right deep down weren’t ventilated to a sufficient level to allow this.”

His comments came as Londoners sweltered on badly ventilated London Underground trains following the hottest September day for more than 100 years.

Tuesday was also the hottest day of the year so far with a national high of 34.4C recorded by meteorologists in Gravesend, Kent.

On the Tube, an even higher temperature of 35C was logged. Transport for London urged commuters to take care and carry bottled water with them.

But rush hour passengers baked on stuffy, sweaty trains. Lyn Luong, a 20-year-old technology analyst from Bow, said: “I think the way I look now is I how I would describe the heat – unbearable.

“I think some people try to avoid the Tube, but if you have to take it you can’t really not, I have to work. Today has been particularly bad and on certain lines it’s worse than others.

Richa Dhingra, 33, from west London, said: “It’s so hot, there’s just no ventilation."

Engineers PriestmanGoode are working with TfL on designs for the "New Tube for London" - set to launch on lines including the Central and Bakerloo between 2025 and 2033.