I Just Learned Why The Tube Really Gets So Hot, And It's Not Why I Thought

Camden tube stationYesterday, parts of London’s Underground network reached a sweltering 33°C, The Standard reported.That’s three degrees above the government’s upper limit for the safe transport of cattle. According to Bloomberg UK, the network of underground trains can be 5°C hotter than the high temperature on the street. To be honest, I hadn’t put much thought into why that might be. Hot day, little air con, and tonnes of people – a recipe for a sauna-level carriage, I reckoned. But it turns out that a good chunk of that notorious heat comes from an unexpected source, which has nothing to do with either the air temperature or the mass of people. Braking heats tube carriages up by more than you thinkIn 2007, a Rail Engineering report found that a lot of the heat comes from braking. That’s because the friction created during the process is very intense, creating a lot of kinetic energy that gets converted into heat energy. Forbes says braking is responsible for about half of the higher heat, while passengers account for 2% of the warmth. Rail magazine puts the combined effects of braking and train motion at 80% of heat input.Then, there’s the ground into which the tube network was built. It’s largely clay and chalk, both of which hold onto heat. This is particularly troublesome for deeper tube lines, which have less access to air. This heat builds up – per Rail Magazine, the average tube temperature in 1900 was 14°C, compared to today’s 20-25°C. Not all tube stations have adequate ventilation, either, meaning hot air has nowhere to escape. The heat simply builds and builds, and because extracted hot air often has nowhere to go, not even air conditioning will provide an adequate solution. What can we do to make the tube cooler?It’s no easy feat. London Underground’s Programme Director for Infrastructure, George McInulty, told Rail magazine: “We can increase the capacity of shafts, and we did this during the upgrade of the Victoria Line in 2011/12.“But we were lucky that it was only built in the 1960s, with half an eye to improving ventilation at a later date and introducing a service of over 30 trains per hour. Elsewhere we’ve been less lucky, and have had to look at retrofitting stuff where we can.” Air conditioning is simply not an option for tunnels that can’t let hot air out; it’ll simply heat some older, hotter lines up more. Some solutions, like turning a lift in St Paul’s into a fan, have helped. In Victoria Station, water from a nearby river is circulated in pipes around the tunnels to keep them cool. But these clever workarounds are not universally available. McInulty said: “We can make gains from deceleration, and we are buying trains with regenerative braking, which produces electrical energy from braking rather than all that heat from friction. “And there are other things that go unseen by the public that make a difference - we have put a thin film on the windows of Central Line trains, so they absorb less energy on the outside parts of the line which is then dumped underground. We’ve also made massive steps in using LED lights which give out less heat.” Still, it’s hard to see how the tube will cool down significantly any time soon.Related...So THAT's Why UK Heatwaves Feel So Much Worse Than Other CountriesHow To Know If You Have Heatstroke – And What To Do About ItWhat Should You Actually Do When There's A Heat Alert?

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