Much of the heat from a subway comes from the HVAC in the train. Add more venting to the trains that works with the trains movement and use forced air into the tunnels. Costs more but it can be done. Some of the heat comes from acceleration and braking. Make that more efficient with newer electric motors, better designed tracks, better regenerative braking. If that doesn't work require everyone to disrobe before leaving their underground home.

Another option would be to deprecate trains. Instead people travel through pressurized maglev modules in tubes at 800 mph / 1200 kph. Transcontinental tubes accelerate the modules to 8000 mph / 12,000 kph. Interplanetary offramps eject the module from earth at mach 50+ where it is caught by a sub-orbital robot ship that matches speed and takes the module to the interplanetary colony ship.