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 th

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.