I learned recently that rail (especially continuously welded rail) is generally kept under tension (I don't understand how it is secured at the end of the line,

I learned recently that rail (especially continuously welded rail) is generally kept under tension (I don't understand how it is secured at the end of the line, though). So when it heats up in the summer, it is simply under less tension. Unless it heats up too much, and then it buckles and you get derailments.