It's Official: The Bakerloo Is The Hottest London Underground Line

By James O Malley on at

Ahhh, look at the lovely weather. It must be summer in London at last. Time to stroll through Hyde Park, stop for a drink at one of the capital’s countless drinking establishments… and then risk death by melting, by stepping on to an unbearably warm Tube.

The problem is that though the Tube is brilliant, it is ill-equipped for the heat, because air conditioning wasn’t really a thing when the tube was built a century ago. So as any Londoner will tell you, this means that it can get really, really hot.

But exactly how hot? And which line is the hottest? A couple of years ago our pals at the excellent CityMetric assessed the below map, which was released by TfL. It shows the tunnelled part of the Tube network, coloured by how warm it gets. At the time, they reckoned the Central Line was the hottest line.

But today, thanks to a freedom of information request we’ve got the actual data - and more recent, up-to-date, data on tube temperatures. Here’s everything we learned. (All temperatures are in celsius because we’re in a civilised country and not a 1950s Brexit dreamland just yet.

The Bakerloo Line is the Hottest Tube Line

TfL released to us temperature data up to the end of 2016 - here’s the table for all of last year, which we’ve coloured in to show relative temperatures. “W&C” obviously stands for the tiny and often forgotten Waterloo & City line, and “SSL” refers collectively to the subsurface lines - the Circle, Hammersmith & City, District and Metropolitan Lines, which together share a lot of track so are treated as one.

So which line is the hottest? During 2016 the single hottest recorded temperature was in August on the Bakerloo Line, when it reached a scorching 31.04 degrees. The Central Line, by comparison, which we would intuitively have assumed to be hottest based on the lived experience of squeezing into a baking carriage at Holborn during rush hour, tops out at 30.47 degrees in August.

Both lines were over two degrees hotter in August than the Victoria and Northern Lines (the latter of which had its high in September - was there a heatwave limited exclusively to Clapham?).

Whether this is a statistically significant difference is unknown as the FOI didn't tell us the methodology. But hey, these official figures make for some nice graphs - and certainly give an indication of just how warm it gets on each line.

Average Temperatures

So what about if we average temperatures over the year for each line? If we do this, the Bakerloo still comes out as the hottest line - with an average temperature throughout the year of 26.955 degrees.

As you can see, the subsurface lines are significantly cooler than the deeper lines, and this is probably due to a number of factors: The tunnels are larger, with more breathing space, they sit just below the ground having been built using “cut and cover”, and they use brand new rolling stock (which might generate less heat?). It isn’t clear whether the data was collected by someone sat on a train with a thermometers or with some sort of device in the tunnel - if the former, then this cooler atmosphere could also be attributable to the fact the newer trains have air conditioning.

We can also average each temperature across each month, and to see how the average temperature of the Tube compares month to month, as you can see above. Though given that this weights each line the same means it could be a little misleading in terms of absolute figures - as the Waterloo & City counts just as much in these averages as the massive and dual-tunnelled Northern Line. Though it does point towards the thrilling observation that - yes - it is hotter in the summer! And perhaps marginally more interestingly, that September on the tube is hotter than July.

We can also use the data to see which lines are impacted most by the seasonal weather. This chart was made by taking the lowest and highest recorded temperatures and working out the difference. Here again the subsurface lines fluctuate the most, and we’d wager this is simply a function of them being closer to the surface - the deep level lines are able to keep heat trapped in during winter much more effectively.

Will It Get Better?

The above data is from 2016, and now you’re probably thinking the obvious thing: Is it going to be as stifling this year, and what can be done?

The bad news is that making the Tube cooler is really hard, because of the aforementioned Victorian engineers. But TfL are slowly working on improvements. The trains on the Bakerloo Line were first used in 1972, and are due for replacement in the mid-2020s. Yes, this means that they will be around 50 years old. The new ones, TfL promises, will be air conditioned like the modern trains on the subsurface lines.

The other good news is that today's modern tunnellers have new technologies and have learned from the problems of previous generations. Just this month the first Elizabeth Line trains are entering service on the line that is currently known as “TfL Rail”, and these are all air conditioned. When Crossrail opens properly, and the central London tunnelled section opens in December 2019, these trains will be used there. And better still, the new tunnels are absolutely enormous and AC has been considered from the start.

So yes, it’s going to be another sweaty summer on the Tube. But hey, at least now you know to especially avoid the Bakerloo Line.

And that’s more than you probably ever needed to know about just how warm it is on the Tube.

Read More: We went behind the scenes of the Elizabeth Line construction at Liverpool Street. Here's what's going on underground.

James O’Malley is the Interim Editor of Gizmodo UK and tweets as @Psythor.