Air conditioning in a busy 19th century underground tunnel system is absolutely an achievement.
Air conditioning an underground train means pumping that hot air into the tunnels. The tunnels have insufficient ventilation because this wasn't conceived of as a problem when many of them were built, and that's a challenge to rectify.
There's also the issue that the trains are already making the tunnels too hot. 4000 trains running back and forth generates a lot of heat from both the engines and braking friction. That heat goes into the clay of the tunnel and is very slow to dissipate due to the high heat capacity of the clay. The trains are never stopped for long enough for the temperature to drop, and so the temperature of the clay has slowly risen 5-12 degrees depending on the line from when the first line was dug in 1863.
Air conditioning the carriages of the new trains didn't result from any developments in air conditioning technology, but from breakthroughs reducing the thermal output of the engines and brakes. This gives them a heat "budget" letting them air condition the carriages without outputting any more heat into the tunnels than a standard train.
Air conditioning an underground train means pumping that hot air into the tunnels. The tunnels have insufficient ventilation because this wasn't conceived of as a problem when many of them were built, and that's a challenge to rectify.
There's also the issue that the trains are already making the tunnels too hot. 4000 trains running back and forth generates a lot of heat from both the engines and braking friction. That heat goes into the clay of the tunnel and is very slow to dissipate due to the high heat capacity of the clay. The trains are never stopped for long enough for the temperature to drop, and so the temperature of the clay has slowly risen 5-12 degrees depending on the line from when the first line was dug in 1863.
Air conditioning the carriages of the new trains didn't result from any developments in air conditioning technology, but from breakthroughs reducing the thermal output of the engines and brakes. This gives them a heat "budget" letting them air condition the carriages without outputting any more heat into the tunnels than a standard train.