The costs associated with building a rail line within an Interstate Highway right-of-way is the major variable. Placing the tracks in the median is cheaper than placing the tracks off to one side because of all the bridges required for on and off ramps. In rural areas whether there is room for one or two tracks is variable, HSR operations require two tracks. The only way to guarantee room for two is to build the Interstate Highway with the room reserved for the tracks.
For examples, IH-4 between Orlando and Tampa reserved space for two tracks when it was rebuilt, IH-25 between Santa Fe and Albuquerque has enough space that the Railrunner laid just a single track. Few private railroad companies will be willing to rebuild half a freeway along with the new tracks, that would be a government supported job. Although Brightline is placing tracks on one side, the freeway and tracks are most of the way in a swamp, which would require a bridge or viaduct anyways.
In urban areas, there is even less room for a new two track rail line as is because all of them have been expanded adding more lanes consuming most of the right-of-way, just about all the freeways would have to be rebuilt to make room for any new rail tracks.
The recently released DEIS for the Atlanta to Charlotte regional rail services shows the costs vs maximum speeds between the three main all well, (1) existing railroad corridor, (2) existing freeway corridor, and (3) new green field corridor. There is a separate thread on it, you might find it an interesting read.
