People take part in ‘No Trousers on The Tube Day’ (not in protest against the heat) on the London Underground in January 2017 JUSTIN TALLIS/AFP/Getty Images

London, it is hot. And nowhere is it hotter than on the sweltering, stinking hellscape that is the Central line. But why, of all of London’s Tube lines, is the Central line so hot and hellish? Simply put, it’s old, it’s very deep underground and TfL hasn’t invested much money into cooling it down. Yet. And, you won’t be surprised to learn, cooling the Tube down is seriously complicated and expensive.

On a network of horrendously hot tunnels, the Central line consistently breaks records. Last summer, Londoners took grim amusement when temperatures on the dreaded red line (35.5C) surpassed the EU limit at which it is legal to transport cows, sheep and pigs (30C). It shouldn’t come as a surprise – the Central line routinely reaches average temperatures too hot for EU cows to handle: 30C (July and August 2013), 30.5C (July 2014) and 31.04C (August 2016). Only the Bakerloo line (30.91C in September 2016) has recorded a monthly average temperatue in excess of 30C during that time. For a larger version of the graphic below, click here (PDF).


All of London’s hottest Tube lines are underground. The Central, Bakerloo, Piccadilly, Northern, Waterloo & City, Victoria and Jubilee lines all run underground through central London. By comparison, the Circle, District, Hammersmith & City, Metropolitan, London Overground are either above ground or relatively close to the surface, which makes getting hot air out and cold air in a cinch. More on that later.

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Average temperatures on the London Underground from 01.08.2015 to 30.08/2015 between 16:00 and 19:00 Transport for London

Cooling the Central line in particular presents an almost impossible puzzle for TfL to solve. In the late 1800s and early 1900s, when many of London’s tube tunnels were carved out of its subterranean clay, engineers didn’t leave a lot of extra space. In fact, they left none. That makes installing air-con units on trains that run through deep level tunnels impossible. The tunnels are too small to allow the heat to escape, effectively turning the Tube into a giant underground oven. Gaze absent-mindedly out of the window on the Central line and you’ll see cables and panelling whizzing past just inches from the train itself. Above them, London’s cloying clay keeps all that heat locked in.


And that clay has been heating up. When much of central London’s Tube network opened in the early 1900s, temperatures in tunnels and at stations were recorded at around 14C. But with nearly 80 per cent of energy dissipated by trains, people and related infrastructure seeping out into London’s clay, it’s been slowly heating up. So much so that the ambient temperature of the clay is now between 20C and 25C. Unlike the Victoria and Jubilee lines, London’s oldest tube lines, and the Central line in particular, suffer from having very few ventilation shafts. And with the Central line cutting a path through some of London’s most densely-populated and expensive postcodes, there are few options for introducing shafts now.

So if you can’t get the hot air out and it’s almost impossible to get cool air in, what can be done? Turns out quite a lot. In 2015, TfL introduced a fan cooling system at St Paul’s Tube station. The system, which was designed by London Underground’s crack cooling squad, pulled fresh air from the street and pumped water around pipes at a rate of 16 litres per second to cool the air by seven degrees before pumping it out at platform level. As a result, St Paul’s has gone from one of the hottest stations on the Central Line to one of the coolest.

To understand how more of the Central line could be cooled down, just look at the Victoria line. In 2006, a similarly experimental groundwater cooling system was installed at Victoria station to lower temperatures in mid-platform areas. In 2011, TfL completed the installation of 13 ventilation shafts throughout the Victoria line, while the new fleet of trains running on the line allows TfL to operate a regenerative braking system, which returns power to the rails when a train brakes. And when a whopping 38 per cent of heat generation on the London Underground comes from trains braking, that’s a pretty big deal. Air from underneath more recently introduced trains is also now circulated into carriages at head height, improving air-flow while the train is moving.

Similar systems are used on the Northern line, while the Jubilee line benefits from being built in the 1970s, meaning engineers were focussed on not creating another sweltering death tube. Fans, ventilation shafts and air cooling systems all combine to keep the Jubilee line on average 5C cooler than the Central line throughout the year.


But, for the most part, TfL is forced to fix the Tube’s overheating problem on a station-by-station basis. Air cooling units are now in use at both Oxford Circus and Green Park during the summer months, with the former making use of a TfL-owned building above ground to pump in water and the latter drawing water via boreholes from a nearby aquifer.

So while other parts of the London Underground benefit either from investment in cooling technology or the luxury of not being surrounded by rapidly-heating clay with nowhere for hot air to escape, the Central Line keeps on cooking. It’s the perfect, sweaty storm and one that TfL is nowhere close to solving.

Updated 10.07.18, 17:20 BST: The Jubilee line was built in the 1970s, not the 1990s.