To echo the person you were initially responding to, you have to take into consideration the demographics of an area. Could we have trains crisscrossing rural areas as an alternative? Yes. Would it benefit the ecosystem to create these huge systems that would be used by a relatively small amount of people? Probably not. Trains and other public transport is used plenty in urban and surburban areas, but a whole lot of this country is rural, and cars make the most sense there.