I would be interested in an article at some point about the relative economics of steep grades versus tunnels in high speed rail. The reason is that the corridor of immediate interest to me is the one between Colorado Springs and Denver. The old-time rail route bypasses a series of high hills, but when the Interstate highway was built they took a more direct route with a much more severe limiting grade. As a result of the new highway route, development since then has been largely in the highway corridor, so a passenger rail system would preferably follow that route.

I have not been able to find a profile drawing that shows the difficulty, but it is the 22 km section between Castle Rock (1897 m), Castle Pines (1999 m), and Lone Tree (1813 m) on this map. These elevations don't really give the whole story; there is a series of quite steep up-and-down grades on the highway route. The old rail route bypasses the region by heading along U.S. highway 85, northwest from Castle Rock to the Ken Caryl area and then into Denver.

I just noticed that Google Maps now shows the rail lines, using light gray lines with cross hatching if you zoom in close enough. You can see the two parallel rights-of-way in this area, (one built by the Denver and Rio Grande and one by the Santa Fe.