Subway stations get so hot because they’re essentially underground boxes filled with powerful heat sources and very few ways for that heat to escape. Trains generate enormous amounts of thermal energy through braking and motor operation, air conditioning units dump their waste heat directly into station air, and the surrounding soil gradually loses its ability to absorb any of it. In New York City, “feels like” temperatures on platforms have been recorded above 130°F, with stations like Times Square hitting 127°F and Union Square reaching 120°F during summer heat waves.
Trains Are Giant Space Heaters
Every time a subway train slows down, it converts its kinetic energy into heat through friction between brake discs and pads. All of that energy has to go somewhere, and in a subway tunnel, it goes into the air around you. The brake disc alone absorbs roughly 86 to 87 percent of the frictional heat generated during a single stop, with the pads soaking up the rest. Multiply that by thousands of stops per day across a busy system and you get a staggering amount of thermal energy released underground with no sunlight, wind, or rain to carry it away.
Electric motors add to the problem. They produce heat continuously while accelerating trains, and much of that heat radiates into the tunnel. Then there’s the air conditioning paradox: the cooled air goes inside the train car, but the waste heat from the cooling process gets expelled directly onto the platform and into the tunnel. Every degree of cooling passengers enjoy inside the train makes the station a little bit warmer for everyone waiting on the platform.
The Soil Stops Helping After a Few Years
When a subway tunnel is first built, the surrounding earth is relatively cool and acts like a natural heat sink, absorbing warmth from tunnel air. This works well for the first one to five years of operation. But over time, the soil around the tunnel heats up and reaches a thermal equilibrium, a point where it can no longer absorb meaningful amounts of heat. The tunnel wall temperature and the air temperature essentially match, and the earth’s cooling effect flatlines.
Research on this thermal deposition effect shows that the warming pattern in the soil changes shape over time, spreading from a circular zone into an elongated oval as heat pushes outward and upward. After about five years, the temperature field stabilizes, and the soil becomes more of an insulating blanket than a cooling mechanism. This means older subway systems are fighting against decades of accumulated heat baked into the ground itself. The soil surrounding the tunnel leads to a gradual, year-over-year increase in tunnel air temperature that compounds over the life of the system.
Ventilation Was Never Designed for This
Most subway systems rely partly on what engineers call the piston effect: when a train moves through a tunnel, it pushes air ahead of it and pulls air behind it, creating a natural draft that should, in theory, cycle fresh air through stations. In practice, this works poorly. The rush of air from passing trains often exceeds what mechanical ventilation systems were designed to handle, disrupting their operation rather than complementing it. Deep stations and multi-level layouts make things worse by increasing the distance air must travel, creating pockets with virtually no circulation.
Using train-generated airflow to push heat out of stations also has a side effect: it spreads warm, polluted air throughout the entire station rather than removing it. In narrow tunnels and corridors, these gusts can feel strong on your skin but aren’t actually replacing hot air with cooler outside air in any efficient way.
Older Systems Have the Fewest Options
The age of a subway system is one of the biggest factors in how hot it gets. London’s Underground, parts of which are over 155 years old, illustrates the problem clearly. The network has two types of tunnels: shallow “cut and cover” lines built just below street level, and deep Tube tunnels bored far underground. The shallow lines were originally designed for steam locomotives, so they have larger cross-sections and plenty of ventilation openings. London has been able to install air-conditioned trains on these lines because there’s enough space for waste heat to vent away.
Deep Tube tunnels are a different story. The gap between the train and the tunnel wall is so tight there’s barely room to mount air conditioning equipment on the train, and even if you could, there are almost no openings to vent the waste heat. On older lines with very few ventilation shafts, it’s nearly impossible to add new ones because the streets above are packed with buildings, utilities, and infrastructure. The result is a system where the deepest, most heavily used lines are also the hardest to cool.
New York’s subway faces a similar set of constraints. Many of its busiest stations were built in the early 1900s with no mechanical cooling, and retrofitting air conditioning into century-old underground infrastructure is an enormous engineering and financial challenge.
Platform Doors Could Help, but Few Stations Have Them
One proven approach is installing platform screen doors or adjustable platform doors that separate the waiting area from the tunnel. These barriers let transit agencies control airflow more precisely. Research on adjustable platform door systems shows they can dramatically change how air moves through a station: closing the system can increase air inflow through station entrances by two to four times compared to leaving it open. By manipulating adjustable vents in these doors, engineers can direct train-induced airflow where it’s useful and block it where it isn’t.
Many newer subway systems in Asia, including those in Seoul, Singapore, and Hong Kong, were built with platform doors from the start. Retrofitting them into older systems is expensive and complicated, requiring structural assessments of every platform edge and compatibility with multiple train types. A handful of stations in New York and London have begun pilot installations, but full deployment across legacy systems remains years away.
How Hot It Actually Gets
The numbers are striking. A data project tracking New York City subway temperatures found that during a June heat wave, the Union Square L platform hit a “feels like” temperature of 120°F. Herald Square reached 112°F. The Dekalb Avenue L station hit 111°F. In July, the Times Square 1/2/3 stop recorded 127°F, and the 6th Avenue/14th Street L station exceeded 130°F. At one 181st Street station elevator, the “feels like” temperature stayed at or above 90°F more than 90 percent of the time over a five-day stretch.
These are heat index values that account for humidity, which is consistently high underground. Subway tunnels trap moisture from groundwater seepage, human perspiration, and the lack of direct airflow to dry surfaces. That humidity makes the actual air temperature feel far worse on your body, because sweat can’t evaporate efficiently in saturated air.
No Official Temperature Limits Exist
There are no federal regulations setting a maximum allowable temperature for subway platforms or stations. OSHA’s heat standards apply to workplaces and focus on employers’ obligations to protect workers. A handful of states, including California, have heat illness prevention rules that kick in at 80°F, but these cover outdoor work environments, not public transit infrastructure. Subway riders waiting on a platform that feels like 120°F are not protected by any formal temperature threshold. Transit agencies set their own internal guidelines, which vary widely and are rarely enforceable by passengers.