Why the Tube Is So Hot - and Why This Is Harder to Fix Than “Just Add Air Con”

What follows is a long-form, slightly nerdy, data-forward look at London Underground heat: how hot it actually gets (with the best public datasets), why the heat accumulates, what London has tried - including some ideas best left in the “well, at least you had a go” folder - how other cities handle it, what passengers actually say, and what is plausibly coming next.

How hot is “hot”, exactly?

The most useful public numbers are TfL’s platform temperature monitoring for the deep-level lines, published as average monthly temperatures at evening peak. These are not carriage temperatures, and they are not a single “hottest moment”. They are better than anecdotes because they are consistent, network-wide, and tracked over time.

Using the London Datastore dataset (2013-2024), the Victoria line is the outlier in two ways: it is the hottest on average and it has shifted upwards dramatically over the last decade.

In 2024, August was the peak month across every line in that table, with Victoria around 31°C and Central around 30.6°C. Carriages can feel worse than platforms because you add passenger body heat, constrained airflow, and heat released by the train itself. That is why, on a busy deep-level service, the lived experience can be “platform is unpleasant, carriage is a mistake”.

Measurement caveat (important, but not a get-out-of-jail-free card)

TfL’s published figures are platform measurements at specified times and locations. Carriage temperatures and humidity vary more sharply with crowding and train design. If you are thinking, “I have definitely been hotter than that”, you probably have. The dataset does not claim to capture the maximum misery. The point is trend and comparison: which lines run hotter, and whether the baseline is drifting upwards.

The physics of a hot railway in a tight tunnel

There are two parts to the Tube heat story: heat generation (where the energy comes from) and heat rejection (how, or whether, that energy escapes). London’s deep-level lines are strong on the first and weak on the second.

Where the heat comes from

A widely cited breakdown from London Underground engineering work (via CIBSE reporting) puts the heat sources roughly like this:

Modern traction systems can feed some braking energy back into the electrical system. London Underground uses regenerative braking, but it only recovers energy when the network can accept it - for instance, another train nearby drawing power. The remainder still becomes heat. So regenerative braking is a genuine mitigation, but not a full solution.

Passengers: small share of total heat, large share of perceived suffering

The “~2% passengers” figure is about heat added to the tunnel system overall. It can still dominate how the carriage feels. A resting adult sheds roughly 70-120 W as heat. Put 800 people into a train and you have something like 60-100 kW of warm bodies, right where you do not want it. Even if that is modest compared with braking energy across the system, it matters locally - and it is one reason “same platform temperature” can feel radically different at 08:25 and 11:10.

London Clay was a great heat sink, until it wasn’t

Early in the deep-tube era, tunnel temperatures were close to the surrounding ground temperature, often cited around 14°C. London Underground even marketed this in the 1920s with an “It’s cooler below” poster.

The original “design” (in quotes, because it was not designed as a thermal system, it just behaved like one) was: heat produced by trains, absorbed into tunnel lining and surrounding clay, gradually dissipated into the ground. This worked when service levels were lower and the ground was effectively an infinite sink. Over decades, the ground around the tunnels warms. Once the clay has absorbed a great deal of heat, it becomes less of a sink and more of a storage heater that someone accidentally left on.

Ventilation is constrained by geometry, history, and competing priorities. Deep-level tube tunnels are narrow. Vent shafts are limited by what is on the surface, and by what you can physically dig through a dense city. More airflow can help thermal comfort, but it must be balanced with smoke control, dust movement, noise, energy costs, and the “piston effect” - moving trains push air around in ways that can fight your intended airflow. In other words, “just add fans” is not stupid, but it can be disappointingly non-linear.

Deep-level platforms trap heat from braking, motors, and hundreds of bodies - with nowhere for it to go. Photo via Pexels.

A short history of “cooling the Tube”

Why London can’t just do what everyone assumes is obvious

When people say, “Why not air condition the trains?”, they are not being obtuse. Many cities do exactly that. The problem is that air conditioning is not “coldness”; it is heat moved from inside the carriage to somewhere else.

On deep lines, “somewhere else” is usually the tunnel air, unless you have a clever way to reject heat outside. If you cool the carriage by dumping heat into the tunnel, you can make the wider environment worse unless you also improve heat extraction.

There are also pure geometry issues. Roof-mounted AC units - common on modern trains - clash with the Tube’s tight loading gauge. Underfloor AC is possible, but it demands space, maintenance access, and compatible airflow paths. More power draw for AC means higher electrical load, and ultimately more heat somewhere in the system unless efficiency gains offset it. None of this means “impossible”. It means “expensive, slow, and entangled”.

How London compares internationally

A useful way to compare metros is by asking three questions: Are stations enclosed and air-conditioned, or naturally ventilated? Do trains have AC, and how is heat rejected? How old and constrained is the infrastructure?

London cannot replicate the newer systems quickly because it would require extensive retrofits: platform doors, reworked ventilation, and new train environmental systems, all constrained by heritage geometry. And when Londoners say “but Paris manages”, the honest answer is: Paris manages some of it, some of the time, and often via long, expensive renewal cycles, much like London is now trying to do.

The ideas London has tried (and the ones that keep coming back)

It is tempting to frame Tube cooling as a single missing invention. In reality, London has cycled through a fairly standard menu of thermal-engineering options, each constrained by underground reality.

A Move more air

Improve fan reliability, increase ventilation capacity, make better use of shafts and draught relief. It delivers incremental gains and helps avoid the worst peaks, but it struggles to reverse a high baseline when the ground is already warm.

B Cool stations locally

Platform-level cooling at major interchanges like Oxford Circus and Green Park. Fan-chiller systems at St Paul’s, and at a ventilation shaft between Walthamstow Central and Blackhorse Road. The Holborn cooling panel trial - a newer, lower-maintenance flavour of the same general goal. This approach is pragmatic. It does not “solve the Tube”, but it can protect the hottest pinch points.

C Use water as your heat transport

Water-based schemes are attractive because they can move heat efficiently through small pipes. They also fit London’s reality: water already needs to be pumped out of tunnels (tens of millions of litres per day), and boreholes can access cooler temperatures at depth. This is why groundwater-based air handling appears repeatedly in engineering literature and TfL deployments.

D Recover waste heat

A genuinely modern twist: treat Tube heat as a resource. Heat recovery projects capture warm air at ventilation shafts, upgrade it with heat pumps, and feed local heat networks. Academic and applied work suggests the waste-heat resource is substantial - hundreds of GWh per year are often cited. This does not automatically cool the whole network, but it can reduce local tunnel temperatures around extraction points, and it improves the economics by turning “cooling” into “cooling plus revenue or carbon savings”.

E New rolling stock with cooling designed in

This is where the story finally gets more optimistic, because London now has something it has not had before: a deep-level train designed with AC as an explicit requirement.

If you are interested in the broader story of how the Piccadilly line fits into Underground modernisation, we covered it in detail in our piece on when the Tube will be fully automated.

What passengers actually say

If you strip away the engineering, the public conversation is remarkably consistent across years and platforms (pun intended).

Londoners: dark humour, tactical routing, resigned competence

In London-focused forums and threads, you see the same motifs. The Central line and Victoria line are described as “saunas”, “ovens”, or “free hot yoga”. People develop route heuristics: take the bus if it is not raining, swap deep lines for sub-surface where possible, and avoid peak crush if you have any flexibility.

Visitors often fixate on the mismatch between London’s mild surface climate and the Underground’s persistent warmth. People from cities where metro systems are fully air-conditioned describe London’s deep lines as unexpectedly harsh, because they mentally file “cool metro” under “basic urban infrastructure”.

Conversely, people from cities with hot, old metros - New York is the obvious example - tend to say some variant of: “yes, it is bad, but at least your stations are not also leaking, screaming, and 100 years overdue a fan belt”. Which is both a compliment and not a compliment.

The one genuinely funny constant: everyone believes their line is uniquely cursed. Even with data showing Victoria and Central as consistent leaders, every line has its devotees insisting that their commute is the true circle of hell. Heat is the great unifier.

So what happens next?

The most realistic “future mix” is a portfolio: air-conditioned deep-level trains introduced line-by-line as fleets are renewed, selective platform and station cooling at the worst hot spots, ventilation upgrades where feasible, waste heat recovery where there is nearby demand, and operational measures during extreme heat. It is not as satisfying as “one big project fixes everything”. It is, unfortunately, more like how large, old, urban systems actually get better.

Practical advice that is not just “be less warm”

Further reading

Deep dive

When Will the Tube Be Fully Automated?

Driverless trains, Grades of Automation, platform edge doors, and what TfL documents actually say.

Guide

Is the Tube 24 Hours? Night Tube Guide

Which lines run all night, when they operate, and your alternatives when the Tube shuts.

Analysis

Are Lime Bikes Worth It in London?

Pricing, passes, borough politics, and how dockless bikes fit alongside the Tube.