Even the lowest density US states have most of the population in corridors or areas with sufficient density.

E.g. Montana used to have passenger rail through the most densely populated Southern part of the state. That region has comparable density to regions of Norway that have regular rail service. (There are efforts to restart passenger service in Southern Montana)

And it's not like places like Norway have rail everywhere either - the lower threshold for density where rail is considered viable is just far lower.

The actual proportion of the US population that lives in areas with too low density to support rail is really tiny.