In places like Spain with large temp swings, HSR (and possibly even standard railroads with welded rail) uses expansion joints every so often - essentially long overlapping points that are constrained so all the motion is longitudinal to the rail. Standard commercial item. I'm surprised that they aren't used more in the U.S., or if they are they aren't advertised much. I've also seen articles about special joints used to accommodate expansion and other motions of suspension and other long bridges, though those cases usually do require some speed restrictions for safety of both the train and the structure.