Hmm. Doubtless someone has looked into this and concluded that a/c on underground trains is a good idea, but there's the small matter of the laws of thermodynamics (the scourge of the early months and years of my university career, aaaarrrrgh) to contend with.
The relevant point here is that energy cannot be created or destroyed, only moved around or converted from one form into another (so that's why brakes get hot, because they're converting the kinetic/movement energy of a vehicle into heat, sorry if this is teaching everyone to suck eggs...
). Air conditioning can't "destroy" heat, it just moves it outside the train, into the tunnel or station. I can just about see this being ok on the sub-surface lines but would be amazed if it is seriously proposed for deep-level underground lines where the stations and tunnels are already pretty warm in the summer.
