I don't see what's stopping the US from building elevated rail over interstate highways. They already own the rights of way, and the radii of curvature are already pretty large. As a conservative lower bound, they should at least be able to match automobile speeds.

Additionally, those highways typically enter cities, so stations could be built over them, often right in the city center. That does leave details about how those stations are accessed, but this seems minor since everything else has already been dealt with.

I wonder if an executive order could make it happen.