US has tons of very dense areas with lots of places that can be easily reached by modern public transit, should political priorities ever change. Trains don't go everywhere because of a conscious, deliberate and top-down (centrally planned) choice to invest in car infrastructure. Trains of course used to go everywhere in North America, and were economically viable just fine, until cars were artificially made more economically viable. Stuck with cars due to conscious choices of past generations, unsuccessfully looking for external excuses ever since.

There are 19,500 incorporated towns in the United States. 76% of those have less than 5,000 people.