Posted 01 June 2013 - 10:18 AM
No problem - now that I'm my computer and not the iPad, I can explain a little better.
When rail is laid, there is a temperature at which the rail isn't under any stress - it isn't under tension, nor is it being compressed. From that state, if it heats up too much, the rail will want to expand (hot rail gets longer), but can't because of the fixed ends. Eventually, if you keep heating up, you get a "sun kink" where the force is great enough to push the rail sideways out of alignment.
Going the other way, when it gets cold, the rail wants to contract - as it gets colder, the tension in the rail will increase, until it's sufficient to crack the rail.
Generally speaking, cracked rails are "better", as they'll show up in the signal system and trains will get a stop signal. Sun kinks are a different story - because the rail stays intact, it won't show up in the signal system, and the first warning you'll get is when a train sees it - maybe it'll be able to stop in time, maybe not.
With that in mind, down south the rail can tolerate hot weather because it's laid using a higher neutral temperature. If rail were laid in Maine at that same temperature, you'd have cracks galore come wintertime...