Marratxi wrote:

As I suggested some months ago, long low water tankers at front and back of
every train. The one at the rear sprays out a very fine mist of water which
will cool the air and then be drained out, taking the heat with it, just as
with any other water which gets into the underground system. I claim the
prize.

That will only cool the air if the water has somewhere to evaporate to.

Would work if you combined it with loads of extra ventilation shafts.

You will end up with a very damp and not much colder underground. Also
it would probably cause everything to corrode and rot faster.

Tim