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. 

 
 
 
 

What's actually in the UK government’s bailout package for Transport for London?

Wood Green Underground station, north London. Image: Getty.

On 14 May, hours before London’s transport authority ran out of money, the British government agreed to a financial rescue package. Many details of that bailout – its size, the fact it was roughly two-thirds cash and one-third loan, many conditions attached – have been known about for weeks. 

But the information was filtered through spokespeople, because the exact terms of the deal had not been published. This was clearly a source of frustration for London’s mayor Sadiq Khan, who stood to take the political heat for some of the ensuing cuts (to free travel for the old or young, say), but had no way of backing up his contention that the British government made him do it.

That changed Tuesday when Transport for London published this month's board papers, which include a copy of the letter in which transport secretary Grant Shapps sets out the exact terms of the bailout deal. You can read the whole thing here, if you’re so minded, but here are the three big things revealed in the new disclosure.

Firstly, there’s some flexibility in the size of the deal. The bailout was reported to be worth £1.6 billion, significantly less than the £1.9 billion that TfL wanted. In his letter, Shapps spells it out: “To the extent that the actual funding shortfall is greater or lesser than £1.6bn then the amount of Extraordinary Grant and TfL borrowing will increase pro rata, up to a maximum of £1.9bn in aggregate or reduce pro rata accordingly”. 

To put that in English, London’s transport network will not be grinding to a halt because the government didn’t believe TfL about how much money it would need. Up to a point, the money will be available without further negotiations.

The second big takeaway from these board papers is that negotiations will be going on anyway. This bail out is meant to keep TfL rolling until 17 October; but because the agency gets around three-quarters of its revenues from fares, and because the pandemic means fares are likely to be depressed for the foreseeable future, it’s not clear what is meant to happen after that. Social distancing, the board papers note, means that the network will only be able to handle 13 to 20% of normal passenger numbers, even when every service is running.


Shapps’ letter doesn’t answer this question, but it does at least give a sense of when an answer may be forthcoming. It promises “an immediate and broad ranging government-led review of TfL’s future financial position and future financial structure”, which will publish detailed recommendations by the end of August. That will take in fares, operating efficiencies, capital expenditure, “the current fiscal devolution arrangements” – basically, everything. 

The third thing we leaned from that letter is that, to the first approximation, every change to London’s transport policy that is now being rushed through was an explicit condition of this deal. Segregated cycle lanes, pavement extensions and road closures? All in there. So are the suspension of free travel for people under 18, or free peak-hours travel for those over 60. So are increases in the level of the congestion charge.

Many of these changes may be unpopular, but we now know they are not being embraced by London’s mayor entirely on their own merit: They’re being pushed by the Department of Transport as a condition of receiving the bailout. No wonder Khan was miffed that the latter hadn’t been published.

Jonn Elledge was founding editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.