The London Underground is an iconic piece of Victorian era engineering. What started in 1863 quickly became a core piece of infrastructure that would define the modern character of the British capital. It’s grown and changed immensely in the many years that have passed. Sadly, increasing patronage and more trains have created problems that the original designers never envisaged.

Deep in those London tunnels lies an engineering challenge. The Tube is literally cooking itself. Every day, millions of commuters descend into a network of tunnels that have been absorbing heat since the reign of Queen Victoria. Those clay-lined tubes have been soaking up excess thermal energy like a giant underground radiator, and now they’re giving it back with interest. The tunnels are simply too hot, and cooling them down is inordinately difficult.

The Perfect Storm of Thermal Chaos

The Tube’s heat problem isn’t just about one thing gone wrong – it’s about everything gone wrong at once. When Victorian engineers designed these tunnels, cooling wasn’t a major consideration. The tight, compact tunnels were built deep, nestled in the clay beneath London. In the early days, temperatures in the Underground were considered comfortably low.

“The Underground’s the only spot for comfort when the days are hot; it is cooler below.” – London Underground poster, 1926

Originally, the clay surrounding the tunnels sat at around 14°C, acting as a heat sink for the network. However, over the years, with more trains coming and going and more heat pouring in, the temperature has risen. It now typically sits anywhere from 19° to 26 °C. That’s just the earth around the tunnels, though. Air temperatures are worse—hitting as high as 47°C during a 2006 heatwave. The problem has been a continual bugbear of the beloved Tube, with concerns that future heatwaves could see temperatures rise ever higher.

The problem varies depending on which part of the Tube you’re on; some lines are worse than others. The Central Line is worthy of the nickname “The Central Heat Line”, with temperatures historically reaching 35°C. That’s not just uncomfortable – it’s approaching the limit of what the human body can handle efficiently. Much of this is due to the tunnel’s design. Opened in 1900, it featured two compact tunnels buried over 20 meters underground with minimal space for ventilation. It’s one of the so-called “deep-level” lines on the Underground network. Meanwhile, the Victoria line hit 31°C at its peak in 2023, and actually overtook the Central line as the hottest line, recording an average temperature of 28°C last year. Contrast that with the newer Jubilee line, which recorded an average temperature of just 22°C—far more comfortable.

To understand the problem, we need to know where the heat is coming from. A breakdown of heat sources was provided by Rail Engineering in 2007. Trains using their brakes, converting kinetic energy to heat, contributed 38% of the total heat input to the underground. The rest was put down to mechanical sources (22%) and the drivetrain (16%)—because those big electric motors get hot in operation.

As we wrap up for cooler temperatures outside, remember to remove coats whilst travelling to prevent overheating. 🧥

— TfL (@TfL) November 6, 2018

TfL at times has to remind customers that the Underground is warm even when it’s cold outside.

The rest of the heat came from a variety of sources, with train auxiliary equipment and tunnel support systems making up 13% and 4% respectively. The human factor can’t be ignored—each passenger is basically a 100-watt heater on legs. Multiply that by the millions of commuters that pass through each day, and you can see the scale of the problem. Indeed, passengers contributed the final 7% of heat generation in the Tube system. Of all the heat generated in the Tube, 79% passed into the tunnel walls, with 11% going into the tunnel itself. The remainder—just 10%—was removed via ventilation.

While the Tube had been slowly getting hotter for some time, the problem really started coming to a head in the mid-2000s, particularly when the European heatwave hit in 2006. Solutions were demanded, but the Underground wasn’t going to make it easy. The oldest parts of the network presented the greatest challenges, with precious little space to fit additional equipment for cooling. Many lines were simply too tight to allow for air conditioners to be retrofitted to existing trains, for example. Even if they were fitted, there would be a further problem of how to remove the additional waste heat generated from the tunnels, which were already too tight to ventilate effectively.

The quagmire had even prompted then-Mayor Ken Livingstone to put forth a £100,000 bounty for anyone that could solve the problem.  However, it went unawarded. Despite over 3,500 proposals, the Underground authorities found only unworkable or unaffordable solutions, or ones they were already considering.

As you might expect, the problem hasn’t just gone away. Indeed, British media have begun regularly putting out articles on the hottest tube lines each year, as well as updates on what is to be done. Looking ahead, climate change is only going to make this underground sauna more challenging to manage. TfL’s engineers are in a race against time and physics, trying to cool a system that was never designed to be cooled.

Transport for London’s engineers haven’t taking this lying down, however. In recent decades, they’ve thrown a range of complicated solutions at this difficult problem. Victoria Station saw major upgrades, with the successful trial of a groundwater-based cooling system and heavily-upgraded ventilation. On the toasty Central line, engineers realized there was an additional heat input into the system. Trains travelled above ground for part of their route, which would see them heat up in the sun and then bring that energy underground. Countermeasures included installing reflective material on train roofs and solar-reducing films on the windows.

Trials of a new panel-based cooling system have also taken place in recent years at the disused Holborn station, with TfL considering a rollout to various stations after successful trials. The system involves circulating cold water through a curved metal structure. Air is chilled by blowing it through the curved panels and into the station. The system is designed specifically to operate in stations on the deep parts of the Tube network, with an eye to keeping maintenance and operation of the system as practical as possible.

Some Tube lines have been lucky enough to get air-conditioned trains, too. These can be found on the Circle, District, Hammersmith & City, and Metropolitan lines. The modern S-Stock trains run largely on the less-deep sub-surface Tube lines, where it’s possible to easily deal with the hot exhaust of the air conditioning systems. These trains also have regenerative brakes, which turn some kinetic energy back into electricity to feed into the tube network. This cuts the amount of kinetic energy turned into heat, which aids in keeping the network cooler.

The Picadilly line is due to gain air conditioning in 2025, when it abandons its 1973 Stock trains for newer models. Other lines will have to wait longer. Central Line is slated to receive new air-conditioned trains in early 2030, which will replace the aging 1992 Stock models operating on that line. Bakerloo, Waterloo and City, and Jubilee lines are slated to receive upgraded trains “within the next 20 years” according to a Transport for London statement late last year.

Air conditioned trains will help to some degree by cooling passengers on the move. However, those air conditioners will necessarily pump heat out of carriages and straight into the tunnels the trains are travelling through, plus some waste heat to boot. That heat will have to be dealt with one way or another, lest the network heat up further. There’s also the problem that passengers on platforms will still be exposed to high temperatures. Ultimately, both the stations and the trains need to be brought down to reasonable temperature levels. Ideally, the tunnels would be, too, in order to protect any customers that get stuck in a tunnel on a broken-down service. TfL also needs to find a way to bring temperatures under control if it wants to increase services. More trains means more heat going into the system—so it’s important to find a way to pull more heat out, too.

Overall, the problem is still a long way from being solved. The fact is that the London Underground has 11 lines, 272 stations, and more than 4,000 trains. Upgrading all of those at once simply isn’t economically viable. Instead, it appears that Transport for London will keep chipping away at the issue, bit by bit, over the years to come. Ideally, this will outpace any increases in average temperatures brought on by our seemingly-ever-hotter climate. For now, London’s commuters will continue their daily descent into one of the world’s most interesting thermal management case studies. Just remember to bring a bottle of water and some breathable clothing– you’re going to need it.