The question is: does RCTC have the money, even for a minimal job? Public projects aren't Brightline in terms of management and funding sources, and the overhead guarantees that it'll cost at least double what Brightline would have to pay. Unfortunately, that's real life.

Capitol Corridor has wanted to add trains (currently only one RT, to Auburn, plus some buses) east of Sacramento for decades. The whole thing was planned out and engineered 10+ years ago. But UP wants a separate track for Cap Corridor from Sacramento to at least Roseville, but with ample connections so they can use it for (parking) freight trains too. Cap Corridor ultimately didn't have the money (it's still on their list for someday), but not having exclusive use kind of put a damper on things too. The other problems with exclusive use include buying the property (not cheap in an urban area, probably a bit easier in the desert) and paying for maintenance and ops (if UP's using it, they can probably be contracted to maintain it too).

I don't disagree with your view, however. Ultimately passenger trains do need to be separated from freight trains to provide faster and more reliable service. Passenger railroads (and some freight, really) need to be built as essentially freeways. Under US conditions, freight takes priority by the laws of physics in addition to operating practices, and that's not going to change as long as the freight railroad owns and operates the facilities. It works in Europe because the primary focus of the railroads (not just HSR) is hauling passengers, with freight as a side line, so the freight has to fit in, rather than the other way around.