E.g. Utah has a population density of 14km^2, similar to Norway. But like Norway, Utah has most of its population in a small portion of the area - the Salt Lake City–Provo–Orem Combined Statistical Area makes up 82% of the population.
Nebraska has a density of 9.63km^2, but Omaha-Council Bluffs + Lincoln makes up more than half the population (and that's ignoring the Iowa side of Omaha-Council Bluffs)
There are plenty of metropolitan areas like this in the US that may not have the density to justify much of an expansion to rail between them, but that are dense enough that a more rail oriented infrastructure within them would be possible.
And as you point out, this is with current US urban planning. If an expansion of rail is combined with adjustments to urban planning to encourage construction aligned with expanding transport systems, it'd become even more viable (e.g. near automatic planning permission for higher density near stations would probably in itself do a lot)
The evisceration of the passenger rail market was in large part caused by intentional urban planning. White flight moved a lot of people from city centers to suburbans where nary a black person could be found, and urban planners in turn abetted the process by building freeways straight through urban cores and turning every other city block into a parking lot in all but the very largest cities (well, except for LA). Uncoincidentally, the cities that didn't get turned into massive parking lots retained much more functional commuter rail systems than those cities that did.