It is amazing how much tension rails are under all the time. On our live steam park rails we have a combination of aluminum rail (lots of heat growth) and steel rail (less heat growth). The weakest point of expansion / contraction are the switches / turnouts. Night time air temps can be in the 30's day time air temps in the 90's with rail temps even higher. The worst kink I've seen on our track was a couple of summers ago and the kink was VERTICAL and about 18 inches high! Russ