You're explaining why we
don't build useful rail. It seems like there's a really easy first place to look in every major city. Find the biggest interstate, discriminating also by straightness and grade, pick one direction, and start building rail. Divide the other side up into two directions. There are very few places that wouldn't be left with >2 lanes of freeway for each direction, even after halving the right of way. For the few exceptions, they'd have to think a little harder.
In reality, such a series of actions could only exist in a society that got seriously focused on moving away from private auto, and that means density would increasingly be in style. As soon as the NIMBY floodgates came down, property prices would plummet in all but the smallest cores of desirable areas, since it would be readily apparent that you can't build all expensive areas to the level of intensity that justifies insane land values seen in sprawling cities.
Again, we could, but I recognize that we don't. Let's just not say that we can't.