It is true that US rail and wheel profile is different from the European one, but you solve that by changing both (after all, when the German ICE made a demonstration tour of the US 15 years ago, its wheels were replaced), and it is not a function of high-speed or conventional. (After all, as others said, many French TGVs continue their travel to destinations along conventional lines, say Paris-Bordeaux, which is high-speed less than half-way, only until Tours.)

The grinding rate I know about is a measure of maintenance needs: how much of a line has to be grinded a year to correct rail surface errors. As such it has nothing to do with compatibility with conventional lines. (And on some heavily-used TGV lines, the grinding rate can be not just 30% but up to 50%.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}