I think I have only been on this forum for two weeks, so I don't think I should get too far into my labyrinthine views about the demographics and settlement patterns of the United States, but I believe that there are two different issues being discussed here:
First is rural and (this is the technical USDA term
FAR, or frontier and remote America. These are areas where mass transit, either within a city, or between cities, is not always feasible because of low populations or population densities.
The second is suburban and exurban America, and even big parts of what could reasonably be considered urban America, and the fact that there is just a cultural attitude there that transit, either intracity, or intercity, isn't present. These cultural attitudes either are caused by a lack of infrastructure, cause the lack of infrastructure, or both.
So there is one question about why Butte, Montana or Bismarck, North Dakota or Amarillo, Texas don't have train service. Those are small places that are distant from other places. There are real logistical difficulties there.
But it is another question as to why there isn't daily train service in Cincinnati or Indianapolis or Houston, or why there isn't any train service in Nashville, or why there isn't any train service between Little Rock and Memphis, or Indianapolis and St. Louis, or Cincinnati and Cleveland, or Atlanta and Memphis...all places where the population density and terrain would seem to be very easy to be connected by rail. (Or, for that matter, why isn't mass transit popular in most of those cities?)
So to me those are two different questions: I am not going to wonder why someone in Pocatello, Idaho doesn't consider mass transit to be a viable option. But I do really wonder why people in suburban/exurban areas consider private vehicles and air travels the only "real" form of travel.
I hope that didn't get too rambly there.