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. 

 
 
 
 

The number being helped by Help to Buy is at a record high. That’s not a good thing

Help to Buy, Bristol, 2013. Image: Getty.

James Brokenshire the secretary of state for the Ministry of Housing, Communities and Local Government (MHCLG) has been having a busy week. On Monday, the department published its rough sleeping strategy, while Tuesday saw the release of the long awaited social housing green paper.

Neither has filled commentators or the housing sector with much confidence – not least because they offer no new support for genuinely affordable homes. This is a problem because, well, there aren’t enough and building more will cost money. 

Since 2010, affordable housing funding has taken a significant hit. Financial support for building social rented homes, where rents are linked to local earnings, has been cut in favour of rents set at 80 per cent of market rents. As a result the supply of new homes at social rent has dwindled dramaticallyIPPR research has shown that, across the country, the so-called affordable rents which have replaced them are simply too expensive for many households on low incomes.

This lack of a support stands in marked contrast to government’s approach to homeownership, as figures released today on sales through the Help to Buy show. Help to Buy loans, in which government lends a household 20 per cent of the value of a new build home (40 per cent in London), are at a record high. To date, around 170,000 homes have been sold through the scheme.

But the Help to Buy policy has two key problems. Firstly, far from supporting those who otherwise couldn’t afford to buy, the scheme is instead assisting households who would be able to buy at some point without support; lower income households continue to be priced out. The government’s own analysis shows that many of those buying through the scheme would have been able to buy at some point in the future without the scheme.

At the same time, the data released today shows that more than a third of those who have made use of Help to Buy to date have household incomes of £50,000 or higher. Around 1 in 10 of those who have bought through the scheme have a household income of over £80,000.

Secondly, far from improving affordability, Help to Buy worsens it. Research by the housing charity Shelter has shown that, through boosting demand for scarce housing, Help to Buy has inflated house prices. 


The main beneficiaries of this are the developers who have factored the scheme into house prices and as a result have seen a significant increase in their profits. Take, for example, the excessive bonus paid to the CEO of Persimmon Homes earlier this year of £75m (originally £100m), which stoked such shareholder and public fury: that was linked to a surge in profits driven by the Help to Buy scheme.

Both of these critiques demonstrate that government is currently not putting its balance sheet to best use. To date, Government has leant out £8.9bn through the Help to Buy. In 2017 alone, the money could have been used by councils to build somewhere in the region of 22,000 homes for social rent, over 400 times the amount actually built in 2016-17.

Letting councils borrow to build affordable housing would make significant steps towards delivering the 145,000 affordable homes which are needed each year, tackling poor housing conditions, over-crowding and poverty. At the same time, it would generate a return for councils, which could in turn be used to invest in more affordable homes and improve existing ones.

Yet, our absurd national accounting rules currently prevent this from happening. Help to Buy lending is not counted towards the deficit. This is because, asGeorge Osborne boasted when launching it, the Help to Buy scheme is a financial transaction, and therefore the taxpayer would be “making an investment and getting a return”.

By contrast, under rules imposed by the Conservative Government of 2010-15, councils are subject to a debt cap on what is called the housing revenue account. This prevents them from borrowing prudently against future rental streams to build council homes. This is despite cries from the Local Government Association that councils want to get building in order to tackle the housing crisis.

On a more positive note, the social housing green paper indicates that the government is starting to budge on this. It has already lifted the borrowing cap slightly in areas with pressure on affordability and is using these schemes to test the possibility of going further. 

This is welcome, but as the social green paper shows warm rhetoric on its own is not enough. Government should commit to changing the borrowing rules, lifting the local authority debt cap and phasing out help to buy.

Darren Baxter is a Research Fellow at IPPR he tweets @DarrenBaxter.