Here in London the tubes aren't air conditioned, but are still exceedingly hot. I'm told that new tunnels are cool but over thirty or so years they gradually warm up, and so there's a noticeable difference between the older lines and the newer ones (less so at the moment as we haven't built a new line for thirty years, but that's another discussion). It also causes some interesting engineering challenges as the clay slowly dries and bakes around the tunnels.
