How to Keep Subways and Trains Cool in an Ever Hotter World
The battle against escalating temperatures in urban transit systems is becoming one of the defining challenges of our era. Jonathan Paul, a dedicated researcher at Royal Holloway, University of London, vividly illustrates this struggle through his personal observations. Armed with a thermometer-equipped smartphone, Paul has recorded an astonishing 42 degrees Celsius (107.6 Fahrenheit) in a London Tube station. This isn’t merely uncomfortable; it’s the kind of oppressive heat that compels people to seek immediate refuge in air-conditioned buildings. Yet, deep underground, such escape is impossible, leaving commuters trapped in stifling tunnels amidst the screeching cacophony of passing trains. This dire situation in London is not an isolated incident but a microcosm of a global predicament, as climate change pushes ambient temperatures ever higher, making once-bearable journeys increasingly arduous and, at times, dangerous.
The London Underground, a marvel of Victorian engineering, presents a unique and deeply entrenched cooling challenge. Its network of tunnels, some over a century old, snakes through thick London clay. This dense geological material, while providing structural integrity, has been relentlessly absorbing and retaining the immense heat generated by trains and their braking systems since the tunnels were first excavated. Over decades, this accumulation has turned the underground environment into a massive thermal battery, radiating warmth back into the stations and carriages. The seemingly straightforward solution of simply fitting air-conditioning units to trains, a common practice in many modern systems, is paradoxical here. Such units, while cooling the interior of carriages, would merely dump their warm exhaust air into the already overheated tunnels, exacerbating the problem rather than solving it. This perpetual cycle of heat generation and retention demands innovative, system-wide solutions that go beyond conventional approaches.
Jonathan Paul, however, has conceived an ingenious solution that taps into one of London’s most abundant, yet often overlooked, resources: groundwater. "Water, as a refrigerant, can hold huge amounts of heat," he explains, adding, "It’s everywhere beneath London." His research focuses on developing a technology that would harness groundwater, typically at a refreshing 10 degrees Celsius, to efficiently ferry excessive heat away from underground stations. This isn’t merely a theoretical exercise; Paul and his team are rigorously testing a prototype of this system deep within a chalk quarry to the west of London, near the town of Reading, meticulously simulating the harsh, cramped conditions of a real Tube environment.
The global scope of this problem is undeniable. As summers become progressively hotter due to the undeniable impact of climate change, ensuring public transport remains not only comfortable but crucially, safe, has become paramount. Reports from around the world paint a concerning picture. Train riders in Japan and Morocco have voiced strong complaints about inadequate air-conditioning during recent heatwaves. A sobering 2023 study from India documented train carriage temperatures soaring as high as 47 degrees Celsius, a level that poses significant health risks. Paul himself has been a direct witness to the human cost of overheating on overground trains, recounting, "I’ve seen four people faint this summer." Such incidents underscore the urgent need for effective cooling strategies across all forms of rail transport.
The concept of air-conditioned trains isn’t new; the first such systems date back roughly a century. A 1933 article, looking back at a time when "everyone dreaded a railway journey in summer," highlighted the novelty and comfort air conditioning brought. If only those early pioneers could foresee the future, where despite technological advancements, trains and underground networks can become so intolerably hot during heat waves that, as a Nature study indicates, many passengers actively avoid using them altogether, opting for less sustainable or convenient alternatives. This reluctance to use public transport due to discomfort poses a significant challenge to urban mobility and environmental goals.
Paul and his colleagues are confident in the viability of their solution. Descending 20 meters down a ladder into the chalk quarry near Reading, one enters their subterranean laboratory. Here, multiple galleries of varying sizes, carved into the chalk and separated by doors, allow them to create controlled environments. "We’re trying to simulate real-life conditions in the Tube," Paul states, acknowledging the somewhat austere setting: "It’s very dark, it’s quite dingy." In 2022, Paul and a colleague published a detailed paper describing their proposed methodology. It outlines how water from subterranean rivers or aquifers could be pumped into specially designed heat exchangers mounted on the ceilings above subway platforms. Hot air, drawn into these exchangers, would transfer a significant portion of its warmth to the cooler groundwater. The now-cooled air would then be recirculated into the platform area, while the warmed water would gently flow away through the ground, potentially to be naturally cooled or treated elsewhere before re-entering the system.
A working prototype of this system is now operational at the chalk quarry. Paul reports promising initial results: "For nominal pumping rate over the course of about an hour we can shift the temperature of the [bedroom-sized] room down about 10-11 degrees [Celsius]." While this demonstrates significant potential, the team still needs to scale up their tests to assess performance in larger, more complex station environments. A crucial next step involves securing buy-in from Transport for London (TfL), the body responsible for operating the Tube, whose implementation of such a system would be pivotal.
Paul argues that his system represents a substantial improvement over a similar groundwater cooling technology tested by TfL in 2006, which is no longer in use. That earlier attempt utilized groundwater that had leaked into, and subsequently been pumped out of, Victoria Tube station. Paul suggests that this "leakage water" would likely have been warmer and less consistently cool than water directly tapped from nearby aquifers or subterranean rivers. Furthermore, his system incorporates specialized filters designed to mitigate the common problem of limescale buildup and blockages caused by chalky water, a critical enhancement for long-term operational efficiency.
When WIRED approached TfL regarding Paul’s system, the agency declined an interview but provided a statement through spokesman Melvin Lim. Lim emphasized TfL’s need to "carefully prioritize" investments in recent years, highlighting the upcoming introduction of new, air-conditioned trains for the Piccadilly Line next year. He affirmed, however, that TfL "stay[s] open to measures that will help manage the impact of increasing temperatures due to climate change." Indeed, TfL has explored various cooling solutions over the years, including trials in 2022 involving cooling panels attached to tunnel walls, which circulated water to remove heat from the air. These panels, while conceptually sound, are not currently in use, with Paul suggesting that such a system could be prohibitively expensive for widespread deployment.
Hassan Hemida, an expert from the University of Birmingham, acknowledges Paul’s water-cooling technology as a "good idea," but echoes concerns about its real-world effectiveness in a bustling Tube station teeming with people, where heat generation from human bodies and train movement is constant. Hemida also highlights the extreme challenges posed by certain high-speed railways. He cites the example of super-high-speed trains, traveling at speeds of, say, 400 kilometers per hour. Such velocities create significant aerodynamic effects, forcing air out of their path at high speeds. This can cause a substantial drop in air pressure surrounding the heating, ventilation, and air-conditioning (HVAC) equipment mounted on the train roofs, potentially making it impossible to draw sufficient air into the HVAC system and leading to system failure. Hemida notes that he has been contacted by colleagues in China seeking solutions to this precise problem, underscoring the global nature of these advanced engineering hurdles.
Despite these challenges, the trend towards incorporating air-conditioning systems as standard in new rolling stock is undeniable and increasingly rapid. London’s relatively new Elizabeth Line, for instance, features state-of-the-art air-conditioning. Skoda Transportation, a German tech company, recently rolled out air-conditioned metro trains for Sofia, Bulgaria’s capital, with a spokesman confirming, "Generally, every vehicle we produce now is equipped with AC." Sharon Hedges, senior engagement manager at Transport Focus, an industry watchdog, underscores this shift in expectation: "As people think about procuring new rolling stock, these are the kind of things that need to be uppermost in minds now."
The demands placed on cooling systems vary dramatically by climate. While British heatwaves present their own challenges, consider the scorching conditions of the Egyptian desert. German tech giant Siemens is currently supplying Egypt with a new fleet of high-speed trains capable of reaching speeds of up to 230 kilometers per hour. Their Velaro trains are proven performers across Europe, but for Egypt, Siemens undertook rigorous testing. Last summer, a Velaro train was subjected to extreme conditions at a test facility in Austria, enduring temperatures as high as 60 degrees Celsius and high winds. Björn Buchholz, head of HVAC and door systems at Siemens, proudly reports, "We are achieving 26 degree inside temperature at the hottest outside conditions." This remarkable performance is partly due to a specialized filter system. "We added a special filter system," Buchholz explains, "to help remove sand and dust entering from the Egyptian desert, to keep the AC operating as designed," ensuring the system’s longevity and effectiveness in harsh environments.
Beyond active cooling systems, passive measures also have a significant role to play, according to John Lawrence, a consultant for the rail industry at JPL Diversified. Many train stations already incorporate awnings or canopies, which provide crucial shade for platforms and carriages while passengers are boarding. However, this isn’t universally applied. Lawrence advocates for maximizing these opportunities: "[The train] can be sat there quite merrily baking in the sunshine – take the opportunity to get rid of some of that solar gain."
Once a train is operating in the open, highly reflective paints or coatings could offer further thermal protection. In the UK, the Rail Safety and Standards Board (RSSB), an independent standards and research organization, is planning trials for a selection of such technologies on trains next year. Richard Walker, deputy director of research at RSSB, expresses optimism: "There is a decent chance that some kind of wrap or reflective coating becomes a cost-effective approach for dealing with this." His colleague, Scarlett Hayward Mitchell, a research support analyst, adds that different trains might require tailored solutions depending on their typical routes – whether they travel north-south or east-west – as this orientation could significantly affect which parts of the vehicle are most frequently exposed to intense sunshine.
Implementing any significant changes to railway infrastructure or rolling stock is, invariably, an expensive undertaking. Consequently, updates to air-conditioning systems or exterior liveries are most likely to occur during scheduled overhauls or when train operators procure new rolling stock. In the UK, a proposed program to transfer some rail services into public ownership, forming a new entity called Great British Railways, could present a timely opportunity to invest in new paintwork or liveries for trains. This potential alignment of policy and need is partly why the RSSB has chosen to prioritize research into reflective materials at this moment, as Walker explains.
Meanwhile, Jonathan Paul and his colleagues remain dedicated to refining their underground cooling technology at their chalk quarry test site. "We’re building slowly in order to present a case that this can be rolled out operationally," Paul asserts, estimating, "We’re maybe a year away from it." The vision is clear: to develop a robust, scalable solution that can genuinely transform the passenger experience in subterranean transit. If Paul, or any innovator, can successfully garner the interest and support of crucial bodies like TfL and effectively cool down the London Tube – an environment notoriously resistant to conventional cooling methods – then perhaps anything is possible in the ongoing global effort to adapt our vital infrastructure to an ever-hotter world. The stakes are high, not just for passenger comfort, but for the sustainability and resilience of urban public transport systems worldwide.
