Calvin Barrows and Sylvia Telatycka

Why is London’s Tube so hot?

The challenge for the rail industry today, TfL in particular, is to figure out why London’s Tube gets so excessively hot in the summer.

There is an indisputable, direct relationship (and subconscious awareness) between the London Tube’s chronic overheating and heatwaves; however, I appreciate the direction of my analysis may appear counter-intuitive for many, and this is clearly causing some ‘cognitive dissonance’ in our engineering community.

London Overground and Underground trains can be prone to overheating in the summer
Soaking up the sun: is solar gain from the Overground carrying heat to the Underground? (Photo by mammuth/iStock)

The first point to consider is that the Tube is a “mixed” over/underground network, which does overheat, whereas wholly or predominately underground-only networks like Glasgow, Warsaw or Prague do not usually overheat. Even allowing for their year-round, operational heat sources, their networks maintain a steady temperature of around 15⁰C

Whether looking at overground-only rail networks or mixed networks, all rolling stock is at the mercy of solar gain when travelling on the surface. Travelling overground in the summer, carriages are relatively easy to cool internally with air conditioning (AC), with hot exhaust air discharging to the atmosphere.

However, when comparing tunnels that are serving an underground-only network with tunnels that are serving a mixed network that has trains travelling both overground and underground, the thermal behaviour outcomes will be significantly different. 

It is important to note that the entire absorbed heat of these latter trains equates to the sum of all the heat sources in an underground-only situation, plus the significant solar gain effects – both direct and indirect. In David Attenborough’s Perfect Planet series, episode 2 at minute 38, he reminds us: The solar energy that strikes our planet in just an hour, contains more power than that used by all of humanity in an entire year.”

What does Transport for London say about Tube overheating?

Transport for London(TfL) and London Underground (LU) have blamed “the brakes” and other operational (year-round) heat sources including passenger body heat. If the sources were only the effects of operational heat, then these would cause overheating in underground-only networks, which they do not. However, although TfL/LU have taken isolated, unrelated snapshots of the overheated trains using thermal imaging, they have not determined whether the increase in heat is truly progressive. 

With a more robust assessment, they may have discovered the prime role of solar gain’s direct and indirect irradiation effects, both on the track and on the trains when travelling on the surface.

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The resulting problem is that not only do the internal surfaces overheat but so do the external surfaces, some by direct irradiation and some, like the undercarriage, by indirect irradiation from the irradiated rails and track ballast. 

How underground tunnels affect tube temperatures

Using AC in the internal carriage space does not address heat re-irradiated from the external train surfaces into the tunnel but does add to the overall tunnel heat because of the heated air the AC exhausts. The best way to deal with this problem is to stop the trains from getting hot in the first place – both inside and out.

Obviously, the traction motors and brakes become considerably hotter in the seasonally hot weather, not just because of their operation but because they have been super-heated in the environment and location in which they find themselves – while on the surface. 

TfL/LU’s own data confirms the seasonality of overheating, as does the clay temperature graph from Cockram and Birnie's The ventilation of London’s underground railways. Moreover, while TfL/LU are constantly proclaiming that climate change is certainly going to make matters worse, they steadfastly resist the premise that the current climate may be implicated.

[Read more: London heatwave: How will the city’s services be affected?]

As a consequence, and despite the numerous, untargeted and misconstrued (in my opinion), multi-million-pound mitigation measures past and planned, realistic proposals for the prevention of these huge solar effects are massively lacking. 

That is why this emphasis on “the brakes” has achieved so little improvement, and why regenerative braking has not been effective in reducing overheating either. Moreover, all these thermal conductivity processes outlined above, including the intensifying effect of irradiated ballast, have been largely ignored or overlooked in relation to the management of overheated rails and consequential rail buckling.

Minimal attention has been paid to the advantages of solar reflective measures. For example, the beneficial use of standard white paint on the non-wearing rail surfaces is ~5⁰C, but the use of solar reflective white paint could reduce the excess rail heat by ~15-20⁰C, and even more if the ballast were also protected from irradiation. 

Without sufficient understanding and inclusion of the role of irradiation on the surface, TfL/LU and their academic advisers and consultants have perhaps misinterpreted the evidence and their modelling will always be flawed, massively underestimating the amount of heat being carried into the tunnels.

Despite the modern-day involvement of WSP/Parsons Brinckerhoff (PB) in TfL’s/LU’s Cooling the Tube Project (CtTP), their engineers and others working on this have ignored or (more likely) been completely unaware of PB’s historic mid-1970s findings in collaboration with the leaders of the US rail networks.  

The PB findings in these early iterations of their seminal works on tunnel design refer to surveying many of the rail/subway networks in the US including the south-eastern Pennsylvania Transportation Authority [SEPTA], which reported, very explicitly, that the cars on their Market Street Line were absorbing solar heat while above ground and bringing this heat into the subway. 

Unfortunately, these findings appear to have been forgotten over time. Yet this research and its results highlight a message that must be taken very seriously.

How is TfL addressing the heat on the London underground?

Much of the funds TfL/LU have spent have been on schemes to cool stations. When such high temperatures occur in the station environment, passengers there have a choice – remain or exit to the comparative safety of street level. However, this focus completely misses the point, because it is the overheated tunnels that are going to harm passengers if their train stalls between stations when the saloon temperatures are 40°C-plus – and similar temperatures have indeed been anecdotally reported on the Tube’s Central Line

In a stalled train though, everyone is totally reliant on railway staff to isolate the current, open the doors, and de-train or otherwise evacuate passengers. Historically such occurrences have often taken hours, not minutes and for many, such a delay could prove deadly.

In its early life, London’s Tube network was advertised as ‘it’s cooler below’ because in those days it was an underground-only network. The overheating consequences of the decision to extend it overground to the suburbs could not have been predicted or assessed at that time.

Meanwhile, Warsaw’s underground-only network engineers are in the planning stages to construct one or more further lines. 

If some part of this expansion is overground, in time it will almost certainly create the same problems as the Tube experiences today. Furthermore, the new air-conditioned Piccadilly Line trains are not going to solve the problem, only exacerbate it. Crossrail also will eventually overheat because it is modelled on the New York Subway, which has had saloon air-conditioning for 30 years – and it still overheats.

In most respects, London’s Tube was and is a brilliant piece of engineering. However, it would be so much more pleasant, and safer, if it did not overheat. From the above analysis, if the mid-1970s observations of PB and confirmed by my own research are open-mindedly acted upon, I do not see this overheating as an intractable problem.

Data visualisation by Katharine Swindells.

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