This article on Wired.com explores the growing challenge of cooling subway and train networks in a warming climate. In London, researchers have recorded temperatures reaching 42°C (107.6°F) within the deep tunnels of the London Underground, which run through dense clay that retains heat from train operations and ambient warming. Traditional air conditioning on trains can actually worsen the problem: it expels heat into tunnel systems that already struggle to dissipate it.
One promising approach comes from Jonathan Paul at Royal Holloway, University of London, who is piloting a system that uses groundwater (~10°C) to absorb heat from the tunnel environment via ceiling-mounted heat exchangers. In tests at a chalk quarry near Reading, this system was able to lower room temperature by 10–11°C in about an hour.
Globally, transit networks from Japan to India face similar issues: heat waves have exposed commuter carriages reaching temperatures of 47°C. Passive strategies also play a role: reflective paint on rolling stock, increased ventilation, and leveraging station design to manage solar heat gain. But scaling these solutions, especially in century-old infrastructure, remains difficult and expensive.
Engineers and transit planners should note that cooling systems must address not only the carriage interior but also the entire thermal ecosystem of tunnels, stations, and rolling stock. Retrofitting modern solutions into historic systems presents major design and cost challenges, yet is essential for safe, comfortable transit in an increasingly hot world.