The tech that finally brought air-con to the London Underground

Londoners have long complained about summer temperatures on the tube, and for good reason, Transport for London (TfL) has admitted that the Bakerloo and Central lines can soar above 30C in the height of summer.

But the oldest tunnels were constructed in the Victorian era and were built just 3.6m wide. This leaves little space for the train carriages themselves, never mind additional equipment like air conditioning systems. Excessive heating of the tunnels is another factor that also needs to be taken into consideration.

These hurdles led TfL and its predecessors to write off such a proposal as too costly and complex to install. But now, thanks to new train designs, the engineering team working on the Piccadilly Line refresh are confident that passengers will finally be able to ride in comfort during the summer months beginning in 2025.

“I think it's pretty obvious to say that passengers have complained for years that tube train carriages are too hot. Especially in summer,” said Simon Ford, programme delivery engineer for the Piccadilly Line Upgrade.

“But we have managed to work with Siemens to end up with a train which has both open wide gangways going between the carriages, and has saloon cooling.”


Open wide gangways are features that will be recognisable to Londoners who have used some of the more modern trains on the likes of the Overground or Metropolitan Lines. They allow passengers to move freely between carriages without doors, improving both air- and passenger-flow.

Traditionally, older trains have bogies under each carriage, the frame onto which the wheels are fixed. But the multi-articulated layout of the new trains, which passengers will experience with the open wide gangways, have allowed TfL to effectively share one bogie between two carriages, leaving spare space underneath.

Until now it’s never been feasible to put the cooling equipment underneath the carriage because there wasn’t the space, nor overhead because the tunnels are too narrow, but the extra space provided by the reduced number of bogies will now make it possible.

Space isn’t the only consideration however, as the additional heat being pumped into the tunnels risks overheating passengers on the platform.

Principal systems performance engineer Tony Lightfoot said the team has focused on energy efficiency in order to lower the temperatures of the trains’ other activities.

“We introduced what's called coasting,” he said. “This is where the trains disengage the power and then roll to their braking point, and then they start to brake so that they can stop at the station.”

This relatively simplistic method of energy conservation has more technical challenges than it might sound. The coasting has been automated in line with the schedules so that if the trains are on time, they are free to coast for longer distances before reaching the platform, but if not, they coast less to reach the platform sooner.

Another significant saving was achieved through the use of regenerative braking which converts the braking energy straight into electricity, rather than heat, which can then be fed back into the broader electricity network.


Lightfoot said this makes the new train “far more energy efficient” and this harvested electricity can even be used by other trains further down the line to power their own operations.

He admitted that once the trains are actually in place and running, it may be necessary to install “additional cooling systems”, which could include costly ventilation shafts.

With passenger numbers, summer temperatures, and a whole host of other variable factors yet to be determined, he gave the impression that the air conditioning system would be a work-in-progress. It also depends “on the direction of the capacity improvement that we're looking at” he said, referring to tentative plans to boost the number of trains on the Piccadilly Line to up to 36 an hour from the current schedule of just 24.

But Lightfoot seemed confident that current plans to boost the number of trains to 27 per hour by 2027 is feasible without “any cooling intervention”.

Frequent users of the Central, Bakerloo and Waterloo & City lines can also look forward to seeing cooler temperatures in the future – the trains have been designed to work on these lines too.

Northern Line commuters however, should not get too excited. Despite experiencing the worst overcrowding on the London Underground, with morning rush hour passenger numbers sometimes climbing to 130 per cent of their peak capacity, that line isn’t due an upgrade any time soon.

Tube trains are generally expected to last around 40 years between major refreshes, but with Northern Line trains seeing a comprehensive refurbishment in 1995, another refresh is not expected for at least 15 years.

“Eventually everything has to get replaced, that's not the same as saying, we've got the money, and we're all ready to go. That's just not the way we work,” Ford admitted.

Programme engineering manager Robert Frith said the new trains will begin testing in early 2024 with the aim to remove all of the old models by 2026.

“Once we've done that we can increase the timetable up to the 27 TPH, once it's new trains only,” he said.

Phase two, which is currently unfunded but still is hoped to proceed, will then follow “and it would be a five or six year programme of resignalling and then optimisation of the performance on the railway,” Frith added.

Ultimately he hopes the new trains and other technologies will roll out across the whole system, but “there just has to be a political will to do so,” he admits.

The Covid-19 pandemic has also made the future uncertain. With the mass move to home working, morning commuting on the London Underground may never return to what it once was. But it is ultimately too early to tell what the long-term impact of the pandemic will be on the system.

Passenger numbers have been ramping since April as lockdown measures eased, but they are still only equivalent to 35 per cent of pre-pandemic levels. Frith speculates that numbers could be more evenly spread across the day in future, but it is too soon to tell at the moment.

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