heat rises and cold falls, so the cold air will stay in the subway system. there is no real need for doors/etc at the entrances as long as you're going down.

heat rises and cold falls, so the cold air will stay in the subway system. there is no real need for doors/etc at the entrances as long as you're going down. the subways themselves put off a substantial amount of heat (from friction, motor loss, etc - all inefficiency eventually becomes heat) and for example in london they have problems with heat buildup in the tube system. so air-conditioning the platforms is generally desirable and probably necessary (especially in a more equatorial climate than england).