Which is London’s hottest tube line?

Temperatures on the tube in August 2013. Image: TfL.

So which underground line is actually the hottest?

This is the sort of question that holds only idle interest for most of the year, but becomes pretty much life-and-death during a heatwave. Like, for example, the one we’re in the middle of right now.

This week, especially on Wednesday, it’s predicted to be so hot that some route re-jigging may be necessary. As noted by the Evening Standard last year, parts of TfL’s public transport network, can, in the summer months, break the temperature beyond which its illegal to transport cattle. Which is less than ideal.

But some sections, of course, are hotter than others. Using a combination of hard facts, science, intuition, and a bit of guesswork, we've ranked the tube lines from hottest to coolest – or, from those you should avoid at all costs, to those where there's a passing chance you might be able to breathe. Here they are: 

How we did it

The temperatures on board the tube rely on a lot of different factors: the size of the tunnel, its depth, the ventilation available, and the number of trains and people bustling through it. 

One factor, though, tends to override the rest: air-conditioning. Air-con is widespread across the Metropolitan, Hammersmith & City, and Circle lines, which makes them, as a rule, cooler than the Underground network (it's currently being introduced on the District Line, which makes it a little warmer than the others on average). 

All four are also "sub-surface lines", meaning they're far closer to the surface than the rest of the network, occasionally pass through above-ground stations like Barbican, and are generally just better ventilated. All four lines are also introducing walk-through trains: that, too, will improve airflow through the trains.

Air conditioning is also operational on a fair number of Overground trains. The combination of being above ground and air-conditioning puts the Overground in first place, in our opinion (though on an extremely hot day the disadvantages of being in direct contact with sunlight could outweigh the advantages of fresh air). 

The DLR is next: it, too, operates almost exclusively above ground, and stops so often that heat doesn't have much of a chance to build up. Plus, you get to feel like you're driving the train. But – no air con.

Then, we get to what are called the "deep level" lines.These are where things get really steamy, thanks to the depth of the tunnels and the thinness of both tunnels and trains.

You can't install air conditioning on these trains: there isn't enough space in the carriages, and besides, there's nowhere for the heat to actually go once the air-con tries to release it. You'd just turn the tunnel into a heating pipe.

Ranking these lines took a little more guesswork. First, here's a heat map of central London provided to us by TfL, which shows the average daytime temperatures at the end of platforms throughout August 2013: 

Image: TfL. 

The Jubliee line, despite hosting some of the deepest tunnels in the network, does best out of these lines: as you can see on the map, it manages to stay coolest, thanks in part to fans which cool the stations' ambient temperature during the night. (The fact it's a relative newcomer, a large chunk of which was built in the 1990s, probably helps.)


Next up is the Victoria line, which was actually one of the hottest lines on a 2006 version of the map shown above. Since then, TfL has managed to cool it down using "regenerative braking systems" on its trains, which convert the friction of braking back into the power supply, rather than into heat; as well as the regeneration of the line's 50-odd ventilation shafts. New trains on the Victoria line also ventilate carriages by pulling up air from the base of the tunnel into the carriage at passengers' head height. 

The Waterloo and City line is, to be honest, a bit of a mystery. TfL's map doesn't give much information on its temperature, though platforms at Waterloo and Bank are both quite cool. We suspect, though, that a calculation of average daily temperatures don't mean much on a line which used mostly at commuter times.One final clue is that it runs four-car trains, not six, which could reduce the temperature – so we've given it the benefit of the doubt and put it in fifth place. 

The Northern line was once famed for its roasting conditions, but, like the Victoria line, recent ventilation improvements and new regenerative brake systems have lowered temperatures considerably over the past five years or so. Things still get a little sticky around Charing Cross and Bank, though. 

The Piccadilly and Bakerloo lines haven't had the same cross-network cooling mechanisms put in place, so they're still hitting the high 20s or even 30°C in summer months. A TfL spokesperson told me that high speed fans have been installed at Bakerloo platforms at Lambeth North and Marylebone to tackle the heat, though we're assuming that was after this map was made:

And finally, we come to the big hitter. The Sauna line. The line where 30°C starts to feel positively breezy. Appropriately enough, the hottest line is the red-hued Central line – something you can on the map above, and which is backed up by more anecdotal evidence:

This, presumably, is thanks to the Central Line's deep, low-ceilinged trains, plus the fact that TfL hasn't introduced any coolling mechanisms on the line over the past few years. 

The Central Line's Twitter feed has this helpful advice for travellers on the sweltering trains:

We have some more: avoid the Central line at all costs. Seriously. Science says so. 

 
 
 
 

Infrastructure populism: on the politics of building big, or failing to

When it comes to infrastructure, they’re all all talk. Image: Getty.

It is famously said of Italian dictator Benito Mussolini that at least while he was dragging his country into a war in which over half a million of his citizens died, he was also making sure the trains ran on time. The murdering fascist wasn’t all bad: he did sort out Italy’s railways.

Now, shockingly, it turns out this is all complete claptrap and Mussolini actually did very little to improve the rail system. Popular beliefs to the contrary are all just part of the fascist myth that he built up around himself to validate his governance.

It’s fake news, but the widely believed claim strikes to the core of a bigger issue: politicians hijacking transport as an easy way to connect with voters. Because, despite Chris Grayling’s best efforts, getting from A to B is an aspect of modern life that can hardly be ignored; effective roads are required to keep the population fed and public transport needed to get people to work.

Like Mussolini before him, this is something Donald Trump has recognised. Among the cries of “lock her up” and “bad hombres”, a key part of his presidential election campaign was a promise to ramp up infrastructure investment. Trump was fed up that other countries “look at our infrastructure as being sad”. As someone who has tried to use Amtrak, the US domestic rail service, I’ve got to say I agree with him.

But it’s easy to complain; it’s following through with a solution that’s the real problem. Only 13 per cent of the $1.5trn Trump hopes to raise is going to come from federal purse, with the rest funded by… erm… something else. It’s in this delivery that this went from being a realistic promise of change to just saying what people want to hear.


On this side of the pond, we have a similar problem: Boris Johnson. A politician who regardless of the issue in question will suggest the answer is an absurd, massive infrastructure project in thinly veiled efforts to grab headlines and deflect from any helpful debate.

His stint as London mayor was dominated by such ill-thought out infrastructure projects. The new Routemaster buses and the Emirates Cable Car were a big waste of money. Aborted suggestions include the Garden Bridge Project, which managed to waste another £46m in public money without even being built, and the Thames Estuary Airport.

More recently, on the prospect of a hard Brexit and lorries queuing up the M20, Johnson proposed a road bridge between England and France. To solve the issue of a hard border between the Republic of Ireland and Northern Ireland, he called for another sea-crossing bridge. You get the idea.

So for Johnson, big projects are a cure-all for difficult problems, which can catch the headlines and allow any reasoned criticism to be easily be framed as not “believing in Britain”. 

Unlike Johnson and Trump, the Hungarian President, Victor Orban, has managed to follow through with his big projects. Unfortunately, they have nothing to do with infrastructure. He has instead spent hundreds of millions of Euros on building football stadiums around the country all to the delight of football-mad Hungarians. This, in a country with one of the worst poverty rates in Europe.

In his 2006 study on the future of high-speed rail in the UK, the director of News Corporation and former CEO of British Airways, Sir Rod Eddington, warned against being “seduced by ‘grands projets’ with speculative returns.” His message was intended for politicians but, in a world of Trumps, Johnsons and Orbans, is surely a lesson that should be learnt by everyone.