[serhei]

Many of the existing freight rail corridors are twisty and not designed in a way that works for high-speed rail at all (unless you buy land anyways to straighten out the more extreme curves) [1]. They might work for conventional passenger rail if you could get the freight railroads to play nice and do better maintenance on the tracks.

[1]: typical example -- https://en.wikipedia.org/wiki/Horseshoe_Curve_(Pennsylvania)

[2]: For a detailed example of what's possible & what isn't with existing corridors, you can look at this independent engineer's study of how to upgrade a rail route through Northern California: http://zierke.com/shasta_route/