Continuous steel rails do have problems with large temperature swings. See the link I posted about "sun kinks." They cause about 50 derailments per year. http://www.climatecentral.org/news/climate-change-warp-railr...

Rail lines have more curves which can take up that slack, and the positional tolerances are much more forgiving. Because of its high speed operation the hyperloop is both incredibly straight and incredibly sensitive to kinking.