Articles by Lewin Day

The London Underground is an iconic piece of Victorian era engineering. What started in 1863 quickly became a core piece of infrastructure that would define the modern character of the British capital. It’s grown and changed immensely in the many years that have passed. Sadly, increasing patronage and more trains have created problems that the original designers never envisaged.
Deep in those London tunnels lies an engineering challenge. The Tube is literally cooking itself. Every day, millions of commuters descend into a network of tunnels that have been absorbing heat since the reign of Queen Victoria. Those clay-lined tubes have been soaking up excess thermal energy like a giant underground radiator, and now they’re giving it back with interest. The tunnels are simply too hot, and cooling them down is inordinately difficult.

The Tube’s heat problem isn’t just about one thing gone wrong – it’s about everything gone wrong at once. When Victorian engineers designed these tunnels, cooling wasn’t a major consideration. The tight, compact tunnels were built deep, nestled in the clay beneath London. In the early days, temperatures in the Underground were considered comfortably low.
“The Underground’s the only spot for comfort when the days are hot; it is cooler below.” – London Underground poster, 1926
Originally, the clay surrounding the tunnels sat at around 14°C, acting as a heat sink for the network. However, over the years, with more trains coming and going and more heat pouring in, the temperature has risen. It now typically sits anywhere from 19° to 26 °C. That’s just the earth around the tunnels, though. Air temperatures are worse—hitting as high as 47°C during a 2006 heatwave. The problem has been a continual bugbear of the beloved Tube, with concerns that future heatwaves could see temperatures rise ever higher.
Victoria and Central have been the hottest lines in recent years, according to TfL data.
The problem varies depending on which part of the Tube you’re on; some lines are worse than others. The Central Line is worthy of the nickname “The Central Heat Line”, with temperatures historically reaching 35°C. That’s not just uncomfortable – it’s approaching the limit of what the human body can handle efficiently. Much of this is due to the tunnel’s design. Opened in 1900, it featured two compact tunnels buried over 20 meters underground with minimal space for ventilation. It’s one of the so-called “deep-level” lines on the Underground network. Meanwhile, the Victoria line hit 31°C at its peak in 2023, and actually overtook the Central line as the hottest line, recording an average temperature of 28°C last year. Contrast that with the newer Jubilee line, which recorded an average temperature of just 22°C—far more comfortable.
To understand the problem, we need to know where the heat is coming from. A breakdown of heat sources was provided by Rail Engineering in 2007. Trains using their brakes, converting kinetic energy to heat, contributed 38% of the total heat input to the underground. The rest was put down to mechanical sources (22%) and the drivetrain (16%)—because those big electric motors get hot in operation.

As we wrap up for cooler temperatures outside, remember to remove coats whilst travelling to prevent overheating. 🧥
— TfL (@TfL) November 6, 2018

TfL at times has to remind customers that the Underground is warm even when it’s cold outside.
The rest of the heat came from a variety of sources, with train auxiliary equipment and tunnel support systems making up 13% and 4% respectively. The human factor can’t be ignored—each passenger is basically a 100-watt heater on legs. Multiply that by the millions of commuters that pass through each day, and you can see the scale of the problem. Indeed, passengers contributed the final 7% of heat generation in the Tube system. Of all the heat generated in the Tube, 79% passed into the tunnel walls, with 11% going into the tunnel itself. The remainder—just 10%—was removed via ventilation.
While the Tube had been slowly getting hotter for some time, the problem really started coming to a head in the mid-2000s, particularly when the European heatwave hit in 2006. Solutions were demanded, but the Underground wasn’t going to make it easy. The oldest parts of the network presented the greatest challenges, with precious little space to fit additional equipment for cooling. Many lines were simply too tight to allow for air conditioners to be retrofitted to existing trains, for example. Even if they were fitted, there would be a further problem of how to remove the additional waste heat generated from the tunnels, which were already too tight to ventilate effectively.
Victoria Station has had plenty of attention in recent decades, with TfL installing new cooling systems. Credit: Oxyman, GNU Free Documentation License
The quagmire had even prompted then-Mayor Ken Livingstone to put forth a £100,000 bounty for anyone that could solve the problem.  However, it went unawarded. Despite over 3,500 proposals, the Underground authorities found only unworkable or unaffordable solutions, or ones they were already considering.
As you might expect, the problem hasn’t just gone away. Indeed, British media have begun regularly putting out articles on the hottest tube lines each year, as well as updates on what is to be done. Looking ahead, climate change is only going to make this underground sauna more challenging to manage. TfL’s engineers are in a race against time and physics, trying to cool a system that was never designed to be cooled.
Transport for London’s engineers haven’t taking this lying down, however. In recent decades, they’ve thrown a range of complicated solutions at this difficult problem. Victoria Station saw major upgrades, with the successful trial of a groundwater-based cooling system and heavily-upgraded ventilation. On the toasty Central line, engineers realized there was an additional heat input into the system. Trains travelled above ground for part of their route, which would see them heat up in the sun and then bring that energy underground. Countermeasures included installing reflective material on train roofs and solar-reducing films on the windows.
Trials of a new panel-based cooling system have also taken place in recent years at the disused Holborn station, with TfL considering a rollout to various stations after successful trials. The system involves circulating cold water through a curved metal structure. Air is chilled by blowing it through the curved panels and into the station. The system is designed specifically to operate in stations on the deep parts of the Tube network, with an eye to keeping maintenance and operation of the system as practical as possible.
Subsurface lines have been running S-Stock trains, which feature full air conditioning to keep passengers comfortable. Credit: (c) Transport for London
Some Tube lines have been lucky enough to get air-conditioned trains, too. These can be found on the Circle, District, Hammersmith & City, and Metropolitan lines. The modern S-Stock trains run largely on the less-deep sub-surface Tube lines, where it’s possible to easily deal with the hot exhaust of the air conditioning systems. These trains also have regenerative brakes, which turn some kinetic energy back into electricity to feed into the tube network. This cuts the amount of kinetic energy turned into heat, which aids in keeping the network cooler.
The Picadilly line is due to gain air conditioning in 2025, when it abandons its 1973 Stock trains for newer models. Other lines will have to wait longer. Central Line is slated to receive new air-conditioned trains in early 2030, which will replace the aging 1992 Stock models operating on that line. Bakerloo, Waterloo and City, and Jubilee lines are slated to receive upgraded trains “within the next 20 years” according to a Transport for London statement late last year.
The Picadilly line will see its aging trains replaced with newer air-conditioned models starting in 2025.
Air conditioned trains will help to some degree by cooling passengers on the move. However, those air conditioners will necessarily pump heat out of carriages and straight into the tunnels the trains are travelling through, plus some waste heat to boot. That heat will have to be dealt with one way or another, lest the network heat up further. There’s also the problem that passengers on platforms will still be exposed to high temperatures. Ultimately, both the stations and the trains need to be brought down to reasonable temperature levels. Ideally, the tunnels would be, too, in order to protect any customers that get stuck in a tunnel on a broken-down service. TfL also needs to find a way to bring temperatures under control if it wants to increase services. More trains means more heat going into the system—so it’s important to find a way to pull more heat out, too.
Overall, the problem is still a long way from being solved. The fact is that the London Underground has 11 lines, 272 stations, and more than 4,000 trains. Upgrading all of those at once simply isn’t economically viable. Instead, it appears that Transport for London will keep chipping away at the issue, bit by bit, over the years to come. Ideally, this will outpace any increases in average temperatures brought on by our seemingly-ever-hotter climate. For now, London’s commuters will continue their daily descent into one of the world’s most interesting thermal management case studies. Just remember to bring a bottle of water and some breathable clothing– you’re going to need it. […]

Ever wonder how those scratch and sniff stickers manage to pack a punch of aroma into what looks like ordinary paper? The technology behind it is deceptively clever, and has been used everywhere from children’s books to compact discs.
Most Scratch and Sniff stickers are simple nose-based novelties, though they’ve seen other uses as diagnostic tools, too. As Baltimore Gas and Electric discovered in 1987, though, these stickers can also cause a whole lot of hullabaloo. Let’s explore how this nifty technology works, and how it can go—somewhat amusingly—wrong.
The Science
3M developed the scratch and sniff technology in the 1960s. It quickly gained iconic status in the decades that followed. via eBay
At its heart, scratch and sniff technology involves the microencapsulation of tiny smellable particles, which are then impregnated into stickers or other paper products. Microscopic amounts of aromatic materiale are trapped inside gelatin or plastic capsules, and then stuck to paper. When you scratch the surface, these capsules rupture, releasing their aromatic cargo into the air. It’s an elegant feat of materials engineering, originally developed by Gale W. Matson. Working at 3M in the 1960s, he’d been intending to create a new kind of carbonless copy paper.
Scratch and Sniff stickers soon became a popular novelty in the 1970s. The catchy name was perfect—it told you everything you need to know. A children’s book named Little Bunny Follows His Nose was one of the first widespread applications. Released in 1971, it  was entirely based around the whole scratch and sniff concept. Children could read along and scratch various illustrations of peaches, roses and pine needles to see what they smelled like. The book was reprinted multiple times, remaining in publication for over three decades.
Other popular media soon followed. Pop rock band The Raspberries put a scratch and sniff sticker on their album cover in 1972. Director John Waters would go on to release his 1981 film Polyester with an accompanying “Odorama” card, which featured multiple smells for viewers to sniff during the movie. The concept still resurfaces occasionally, though the gimmick is now well-worn. In 2010, Katy Perry’s Teenage Dream album smelled like cotton candy thanks to a scratch-and-sniff treatment on the Deluxe Edition, and King Gizzard & The Lizard Wizard put a similar touch on 2017’s Flying Microtonal Banana.
Best Intentions
Gas safety education is one of the most common uses of scratch and sniff technology today. via National Energy Foundation
Could scratch and sniff technology be put to more serious and noble uses? Enter Baltimore Gas and Electric Company. In 1987, the energy company had found the perfect way to educate customers about gas safety.  The plan was foolproof—mail out 300,000 brochures with a scratch and sniff panel that would familiarize customers with the distinctive rotten-egg smell of mercaptan. That’s the sulfur compound added to natural gas to make leaks more easily detectable.
The brochures featured a red flame impregnated with scratch and sniff material. “Scratch this flame with your fingernail,” read the mailer. “Sniff it. . . . Let your family sniff it and be sure everyone recognizes the odor.”
The mailers were sent out with the best of intentions, in the pursuit of education and public safety.  Unfortunately, the problem soon became apparent. Paper envelopes aren’t exactly hermetically sealed, and the stickers used were simply far too potent. The microencapsulated mercaptan scent was floating out of the envelopes before anyone could even get to the scratching part. Soon, the smell of gas was wafting out of these brochures all across Baltimore.
BG&E uses a scratch and sniff element in its modern gas safety brochures. They’ve found a way to refine the technique to cause less trouble. via BG&E
The result was exactly what you’d expect when 300,000 pieces of mail start simulating gas leaks all over town. Fire departments across the city were fielding a deluge of calls from concerned citizens who thought their houses were about to explode. Many hadn’t opened their mailers—they’d simply detected the smell and rang in the alarm.
The LA Times caught the story, and reported that Baltimore firefighters had responded to “at least half a dozen false alarms.” Officials noted that one call was attended by 27 firefighters and 8 pieces of equipment, all over a poorly-thought-out brochure. “I finally went up to this BG&E bill on the table, and the odor was so strong, you only had to be in the vicinity of it,” fire Capt. Raymond Devilbiss told the LA Times.
Spokesman for Baltimore Gas and Electric Company, John Metzger, would later describe the faux pas as “somewhat of an embarrassment.” The company quickly withdrew the remaining brochures, but the damage was done. They’d successfully demonstrated that their gas detection additive worked perfectly – perhaps a little too perfectly.
Funnily enough, this incident didn’t discourage other utilities from trying the same thing. Promo Printing Group in Florida produces a range of mercaptan scratch and sniff cards for various cities and gas utilities. You can get them from the National Energy Foundation, too. Utilities are still mailing them out, as well, and there’s at some anecdotal evidence on Reddit that this actually helped someone catch a gas leak in their own neighborhood.
via Reddit
The problem in the Baltimore case seems to be that the scratch and sniff stickers were simply too potent, or were otherwise releasing their scent when they shouldn’t have been. The incident serves as a reminder that even the simplest ideas can have unexpected consequences, especially when you’re literally mailing out thousands of artificial gas leaks. It’s a cautionary tale about the importance of exploring all possible failure modes–even the ones that seem absurd at first glance.
In the end, Baltimore Gas and Electric learned a valuable lesson about the potency of microencapsulation technology, and fire departments across Baltimore got some unexpected drill practice. As for the residents? They certainly didn’t forget what a gas leak smells like anytime soon. Indeed, though, the education campaign might have been pointless for some—the false alarm suggests many residents already knew the aroma quite well! […]