well... we actually dont have any cooling system on those here, just a small amout of vents, the trips are very short here tho' because we have little stations, so the subway dont take you THAT far.
But its dangerous when it fails, because the temperature can get very hot very fast under the sun, so they reccomend you to drink at least 1lt of water before getting in to avoid health issues in the case it fails...
well... we actually dont have any cooling system on those here, just a small amout of vents, the tri