[ardecila]

It's true that interstate ROWs aren't a silver bullet for building cheap HSR, the way transit agencies have used them for cheap light rail.

If the median is not available, or has steep slopes or sharp turns, then the rail line has to go to one side. In most cases this means threading through every interchange ramp and Citgo/Cracker Barrel commercial zone. Lots of expensive flyover ramps to build and expensive property to purchase. In rural areas, land near freeways is usually the highest value land, while land away from freeways is very cheap and undeveloped, with plenty of room to build bridge approaches, etc to keep the rail line grade separated.

In some cases, though, the land along freeways is undeveloped too. This is the case with Brightline along SR-528 in Florida, it is arrow straight across flat land and no development between Cocoa Beach and the Orlando airport. Also, Brightline isn't using the median but a greenfield ROW that is several hundred feet north or south of the freeway. The I-12 alignment between Victorville and Las Vegas is the same. Hundreds of miles of undeveloped desert land with zero development except for 4-5 towns.

Also, even if the median of a freeway is available, wide enough, and features no sharp turns or steep grades, it's still the median of a freeway. It's a highly constrained worksite, surrounded by constant traffic 24/7. Not exactly a cheap or easy place to do major construction.