London is a sauna – and nowhere more so than on the sweltering, sweaty public transport network.
Underground trains and buses whizz people on five million journeys every day apiece. On the Tube there are up to 543 trains running at peak time, with an average max capacity of 900 people, and thousands of buses carrying up to 90 people.
So it’s of little surprise that the passengers get cooked in the summer, particularly in the midst of the current heatwave. ‘Sauna’ is no exaggeration – a group of climate protesters rode the Central line in towels last week.
But to find out just how hot it really on public transport, took a trip on Tuesday in 30C heat – and came away both concerned and sweaty.
Our trusty air thermometer – a nursery monitor shaped like a penguin – found buses were 3C hotter and Tube trains were up to 5C hotter than outside. Experts have warned that these temperatures can make people ill – and are only set to rise.
Setting off from the Mail’s offices in Kensington, we boarded a bus bound for Notting Hill, where our monitor leapt up to 33C, even with the windows open and cool air being blown from vents over our heads.
From there, we boarded the Central line – long regarded as one of London’s warmest Tube routes – where our penguin friend recorded a temperature of 34C.
It was a similar story on the Bakerloo from Oxford Circus: 34.2C, creeping towards 35, but coolest if you were lucky enough to be standing next to the connecting doors with their windows flung open.
As we transferred from line to line, the temperature did take a much-welcomed dip to around 33C within the stations. In Oxford Circus, we found a gigantic fan behind a mesh grate – and cooled ourselves down to around 31C for a few precious seconds.
But it was as we boarded the Victoria line – an up-and-coming new entry in the hellish heat rankings – that we really began to sweat, with the temperature hitting 35.1C. These three lines are all ‘deep-level’ Tube lines, running up to 100ft below ground.
On our return leg to Kensington, we boarded the Circle line – one of the ‘sub-surface’ lines that goes no more than 30ft underground and has air-conditioning. Even then, our penguin friend recorded no lower than 32C.
Back in the office with water to hand we were relatively lucky. We travelled at around 12pm, before the peak of the afternoon, when foot traffic tends to be light. It would be even hotter if we had been packed like sardines on a rush hour train at 5pm.
But even 30C can be problematic for people travelling on public transport when the weather is stiflingly hot. A rapid analysis of the current heatwave conducted last month found almost 600 people could die in the current hot spell.
Lewis Halsey, a professor of health sciences at the University of Roehampton, previously told that temperatures above 30C can be ‘difficult’ for the body to bear, particularly in humid conditions such as in packed Tube trains.
‘Over 30 degrees – we all know its not the most pleasant temperature but it’s not going to kill us off unless we’re incredibly frail,’ he said.
‘But if it gets humid, which of course it can get it London, temperatures above 30C can get difficult for the body to “dump” heat as quickly as we’re taking it on.
‘If the body can’t lose heat as quickly as it’s taking it on, you’re going to get a net gain of heat into the body and body temperature will rise.
‘And if body temperature rises too much then you’ve had it.’
In high humidity conditions and at around 40C, he added grimly, the most vulnerable could ‘keel over’. And the chances of that happening are only set to increase.
In around 1900, the temperature inside Tube tunnels was around 14C. Underground bosses were so proud of its pleasant climate that a 1926 poster boasted: ‘The Underground’s the only spot for comfort when the days are hot. It is cooler below.’
These days the closest measure is taken on platforms: across 2013, it was an average of 22C across all Tube lines. In the last decade, it has risen two degrees to 24C.
On the stifling Victoria line, temperatures have risen by six degrees from 22C to 28C between 2013 and 2024, likely as TfL bosses have increased the frequency of trains. ‘It is cooler below’ wouldn’t hold up now.
The Met Office definition of a UK heatwave is three consecutive days of temperatures above 28C. On some Tube lines, commuters are living through heatwaves every time they hop on board in the summer.
The Tube is hottest on Bakerloo, Central and Victoria line platforms – which are ‘deep-level’ Tube lines as much as 100ft below ground, lower than ‘sub-surface’ lines such as the District and Circle lines which sit around 30ft below ground.
On the Victoria line, the only Tube route to run completely underground save for its Tottenham depot, temperatures sat at a sizzling 30.4C last summer – warmer than Spanish party island Ibiza at the same time of year.
That’s above the 30C legal limit at which the UK Government’s Animal and Plant Health Agency says it is too hot to transport livestock. It makes you question who the real cattle is.
But why is the Underground so hot – and what can be done? In short: a few reasons, largely owed to its age and design; and not a lot.
TfL says it is on a mission to cool the Tube down – and has been for decades.
But it has a Sisyphean struggle on its hands as temperatures continue to rise year-on-year and it contends with the fact it can’t simply rip out a 100-year-old design and start again.
‘As I’ve often said, with the various quirks of the Underground: a Victorian built it, and now we’re stuck with it,’ Jago Hazzard, a pseudonymous YouTuber sharing the history of London transport with a quarter of a million subscribers, tells .
‘(The Underground) is a deep, narrow tube surrounded by clay. There are so many people, there’s the friction of the brakes, the various sorts of electrical machinery on the trains themselves, a whole lot of different reasons that all come together.
‘The deep-level Tube lines are particularly afflicted because there’s so much clay on top of them and so little space around the trains for air to circulate.’
There are a few reasons that Tube bosses have acknowledged for the endless encroachment of heat, and some additional theories posited by experts.
Excluding the sweaty passengers, Underground trains give off a lot of heat – from the motors and the auxiliary lighting systems to the brakes, even following the switch to regenerative brakes that absorb energy that would otherwise be lost as heat.
Then there are the reams and reams of thick electrical cables you can see bolted to the walls of Underground tunnels responsible for powering the signals.
Much of the heat is absorbed by the tunnels and would once have been absorbed by the London clay – the malleable but highly stable soil north of the Thames that was valued by Tube engineers because it was easy to tunnel into.
But studies have suggested the clay has now warmed to the point it can no longer effectively absorb the waste heat from trains.
And with nowhere else to go the heat simply sticks around in the tunnels, from where it is shoved onto the platforms by trains ploughing through at up to 60mph.
Calvin Barrows, a retired civil engineer, has another theory: that of ‘solar gain’, the heat absorbed by Tube trains when they are exposed to heat from the sun, much like your car when left in the sun on a hot day.
This, in effect, turns the trains into ‘mobile storage heaters’, he says, that then radiate and push heat into the tunnels.
Central line trains are painted with solar reflective coatings and films on their roofs and windows, but Mr Barrows – a one-time long-suffering Central line commuter – maintains that more could be done, such as storing trains in covered areas.
‘It’s not the passengers – it’s only when it comes out on the surface that the trains heat up,’ he said.
‘The trains gain their heat on the surface and carry that into the tunnels. Air temperature in the sun and the shade are exactly the same – but stand in the sun and you become warm because you’re being solar radiated.’
Dr Gary Fuller, a senior lecturer in air quality measurement at Imperial College London, believes smog may also play a role, trapping heat.
said: “Bad air pollution often adds to the direct impacts of heat. Air pollutants can react together in the strong sunlight and heat to form summertime smog.
‘This is the type of air pollution that plagued cities like Los Angeles for the second half of the 20th century.’
London’s older Tube lines were also built before ventilation ever had to be considered in the way it must be now – some of them more than 100 years ago.
And now the network is stuck with antiquated designs about which little can be done without tearing the entire city apart. Opening up the ground to add new vents to the Central line, for example, would be almost impossible.
Newer trains are air conditioned: the Elizabeth Line and London Overground were designed with air-con on board, and new-build stations on the ‘Lizzy line’ were designed from the ground-up to be as cool as possible.
The Circle, Hammersmith & City, District and Metropolitan lines received air-con as part of an upgrade programme a decade ago. Around 40 per cent of the Tube network is now air-conditioned – but that leaves 60 per cent still sweating.
But a question remains over whether the New Tube for London trains, which will bring air-con to the Piccadilly, Central, Bakerloo and Waterloo & City lines, will cool passengers at the expense of cooking the tunnels and platforms even more.
Aircon systems need to be able to spew out the hot air that is being diverted away from the carriages, adding more heat to those already toasty tunnels. The first NTfL trains are expected to enter service on the Piccadilly line next year.
‘Putting air conditioning on the trains only solves the problem inside the train,’ adds Calvin Barrows.
‘But the thing that’s making the tunnels hottest is the outside of the train. Air conditioning just sends more heat out.’
There is also another issue that Industrial Era London did not have to consider: between higher-density construction and climate change, the capital is heating up.
Met Office data suggests 30-year average temperatures for July at its Hampstead weather station just north of Camden have risen 1.4C since the 1960s.
But the city centre is even warmer thanks to the ‘urban heat island’ effect seen in city centres across the world where a lack of vegetation, dense buildings and dark surfaces like roads create the perfect environment for heat to take hold.
On average, the centre of London is 4.5C hotter than its less built-up areas, according to a 2023 study by consultancy Arup using modelling from the University of Reading.
‘In Britain, we’ve been so used to it being cold and wet that rising temperatures have rather taken us by surprise,’ adds Jago Hazzard.
‘These issues were just things that, circa 1900s, they just didn’t even consider. What TfL are having to do now is see what they can do within the constraints of the infrastructure they’ve got.
‘Unless you’re going to rebuild stations at a cost of billions, you very much have to work with what you’ve got.’
TfL’s crack team of engineers have been working to find practical solutions to cooling the Tube for more than two decades – and as yet, nobody has a silver bullet.
In 2003, then-Mayor of London Ken Livingstone offered a £100,000 prize for anyone who could present a practical idea. It was never awarded, reported the BBC, which also noted that passengers withstood temperatures of an almost nostalgic 30C.
Instead, TfL is taking a piecemeal approach, using a multitude of methods and experiments to find ways of cooling platforms, venting tunnels and reducing the amount of heat the trains themselves give off.
It’s not uncommon to encounter enormous industrial-sized fans, hidden behind safety meshes, in the summer months.
On the Victoria line, TfL says it has doubled the capacity of its 13 ventilation shafts.
And in 2015, a disused lift-shaft at St Paul’s station was retrofitted with an enormous fan circulating 16 litres of cold water a second around pipes in the ventilation system – cooling air by up to 7C on the eastbound Central line platform.
In 2022, it trialled cooling panel systems that use fans to circulate air cooled by pipes filed with cold water at a disused platform in Holborn station. The trial brought temperatures down between 10 and 15C around the panel.
Last year, Mayor of London Sadiq Khan said in a written answer to London Assembly member Hina Bokhari that they would be tested at Knightsbridge and could be rolled out at Green Park, Holborn, Leicester Square and Piccadilly Circus.
But rolling out initiatives like these are expensive, and TfL only has so much to spend on these projects, often seeking external funding for trials like that at Holborn.
But there’s no getting around the fact that temperatures above ground are continuing to climb, seemingly unabated – and the Tube will continue to cook, at TfL’s expense.
The 40C July 2022 heatwave saw Tube travel drop by 29 per cent across two days – costing it millions in lost fare revenue. Tracks buckled and overhead cables sagged in the heat – issues likely to rear their head again in future.
Carl Eddleston, TfL’s director of network operations, says the transport body has a ‘comprehensive hot weather plan in place’ to keep the network going, and encourages Londoners to carry water and look out for one another.
But that could prove more difficult as the years roll by: Met Office research suggests that temperatures in excess of 35C could happen every other year by the year 2100 – rather than once every five years or so.
Even as it invests in solar energy, as it announced last week, and plants wildflowers to fight climate change, the uncomfortable truth is that the Tube will almost likely never be completely cool.
‘We’re going to kill people,’ Calvin Barrows warns. ‘If you’re in 40 degree heat and the train gets stalled in a tunnel, you will come out in a bad way, if you come out at all.’
‘It’s a long-term thing – there is no simple solution,’ concludes Jago Hazzard.
‘It’s very much a case of what works, what we can install in the existing infrastructure. I fear, unless stations can be rebuilt, it is going to continue to be a problem, at least in the short term.
‘But in terms of a cool Tube? That’s a long-term thing.’