The highest temperature that Jonathan Paul has ever recorded in a London Tube station is about 42 Celsius, or 107.6 Fahrenheit. Paul, a researcher at Royal Holloway, University of London, uses his thermometer-equipped smartphone to take such readings. 42C is the kind of heat that would send someone running to the nearest air-conditioned building. Underground, though, they can’t. There’s nothing but stifling tunnels and screeching trains down here.
The Tube network runs through thick clay, and that dense material has been soaking up heat generated by trains since the tunnels were first dug, in some cases more than 100 years ago. Fitting air-conditioning units to trains risks heating up the tunnels even more, as warm air from inside carriages gets dumped into the aging tubes.
But Paul has an idea to cool the tunnels themselves. “Water, as a refrigerant, can hold huge amounts of heat,” he says. “It’s everywhere beneath London.” He’s working on a technology that would use groundwater at roughly 10 Celsius to ferry excessive heat away from underground stations. And he’s testing it deep in a chalk quarry to the west of London, near the town of Reading.
Trains, being metal tubes packed with people, are hard to cool down. But as summers get hotter due to climate change, ensuring public transport remains comfortable and safe is becoming ever more important. It’s a global issue. Train riders in Japan and Morocco have complained about insufficient air-conditioning during heat waves this year, and a 2023 study reported train carriage temperatures as high as 47 degrees Celsius in India.
Paul, for one, has witnessed the effects of overheating on commuters using overground trains. “I’ve seen four people faint this summer,” he says.
The first air-conditioned trains date back roughly a century. One 1933 article put it like this: “Until now, every one has dreaded a railway journey in summer.” If only they could have seen the future. Today, trains and underground transport networks can be so uncomfortable during heat waves that many passengers avoid using them altogether.
Paul and his colleagues believe their solution will work. Climb 20 meters down a ladder into that chalk quarry near Reading and you will find it. There are multiple galleries of varying sizes carved into the chalk, separated by doors. “We’re trying to simulate real-life conditions in the Tube,” says Paul—though things down in the quarry are a little more drab. “It’s very dark, it’s quite dingy.”