'All possible'? What are all possible temperatures? I think you've answered your own question, if you think about it: a line has to be drawn somewhere, you have to do some sort of statistical analysis, chance of circumstances occuring which this would not cope with <x%, yeah ok good enough, that sort of thing. That's engineering.

That's not the way metal works.

I'm pretty sure it comes down to cost. Steel rail has to meet certain wear and strength requirements as well as be reasonably inexpensive to manufacture. And there is not likely an alloy that will accomplish that and not expand in the heat.

In the US, the typical construction method is welded rail. AFAIK there is no allowance for expansion, at least on straight sections. Perhaps curved sections can move a bit to allow for expansion but move too much and the train goes off the tracks.

Because that's not possible without a lot of expense. Metal expands thats just basic physics. Its hard to stop that.

Not sure why you’re getting downvoted. This seems like an earnest question.