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. 

 
 
 
 

This interactive map of the Swiss rail network is just really, really cool

The first train crossing the Gotthard Base Tunnel, the world's longest rail tunnel during the opening ceremony near the town of Erstfeld, Switzerland, on 1 June 2016. Image: Getty.

After 500 years of democracy all Switzerland has produced is the cuckoo clock: so said Harry Lime, Orson Welles’ character in The Third Man.

This is terribly unfair – so I’m going to leap to the defence of the proud Swiss people, and disprove Lime’s claim right now, by showing you a mind-boggling map of the country’s rail system.

Made by self-described hacker, trainspotter and map addict Vasile Coțovanu, it uses data from the federal railways organisation SSB to show the movement of trains across the country in real time. If you’re too impatient to watch the trains crawl across the map, you can speed the whole thing up. If you feel the need to follow a particular train, you can do that, too.

The map is not actually live, as such: it uses timetable data to show where trains are meant to be, so doesn’t show delays and so forth. But Swiss trains have such a reputation for punctuality that the joke is the locals set their watches to the trains – so we can be fairly sure what we’re seeing is spot on.

The map also, indirectly, shows both the physical and human geography of the country. You’d be hard pressed to find a more iconic duo than Switzerland and mountains: the country’s topography has allowed the small republic to keep out of Europe’s wars for centuries.

But as well as providing a natural barrier that would make Donald Trump go green with envy (what colour does green and orange make?), the mountains have been a huge obstacle for the engineers tasked with building the country’s rail system.

This map shows the rail network in its entirety: note the concentration of red to the north, representing the Swiss Plateau. Hemmed in to the north by the Jura Mountains and to the south by the mighty Alps, this stretch of relatively flat land was an obvious choice for settlers, and although it only covers 30 per cent of the country, two thirds of the total population lives there.

In the rest of the country, though, rail coverage really thins out. Tunnels and difficult spiral climbs are necessary for trains taking on the mountainous south.
Despite its difficulty, the route through the Alps along the Gotthard Pass has been an important trade route for centuries. It is the shortest route between the Po and Rhine Rivers, and control of it has been a key objective for the Swiss state.

Two years ago a new rail line opened along this north-south route, known as the Gotthard Base Tunnel (GBT). The tunnel can be seen in the more faded red, running from Erstfeld to Biasca:

This tunnel was built quite a bit after Harry Lime’s time: he had seen it, I doubt he would have been quite so rude about Switzerland. It’s the longest and deepest rail tunnel in the world running for 57km under the Alps. At its deepest point, the GBT is 2,450m deep. This incredible feat of engineering allows faster and more frequent journies through the mountains and onto Italy.

Here you can see the 19:09 for Como go through the tunnel:

The Alps aren’t just an obstruction, though: where railway lines have managed to wind their way through you can find some of the most beautiful train journeys in the world.

The Bernina Express is the sort of old school Alpine train where you imagine you could find James Bond in the bar carriage. It travels along two World Heritage railways, the Albula and the Bernina, which are among only a handful of rail lines whose importance has been recognised by UNESCO. Sweeping past glaciers and lakes, the train goes through 55 tunnels and across a mindboggling 196 viaducts and bridges.

Here’s the 13.48 to Tirano going through the eponymous Bernina Pass:

I was thinking of asking Vasile Coțovanu, the man behind the map, to try mapping out the UK rail system in a similar way. The problem is the difference between actual running times and those timetabled is so great that any map would be less of a practical tool than a work of utopian fiction.

Let’s leave these interactive maps to a country that can build a world class rail system in the middle of a mountain range. Harry Lime, eat your heart out. 


Images: Vasile Coțovanu.