Nice use of the decimal point there CD..!
Actually after doing a few minutes ceaseless searching, it appears that the problem is in the ballast more than the rails. If we, in the UK used fixed concrete bases for the rails, then it wouldn't be a problem (a la TGV, or the Supertram), but we don't. We still use ballast and that moves in the heat, and allows the rails and sleepers to move in the heat, and that is why we have to slow down. It is a cheaper method, but ultimately isn't as reliable in the extremes as fixed concrete rail beds. Learn something new every day!