>In order to get coast to coast public transport you'd have to take the whole of the country (the United States) into account and then you'll find that that average population density suddenly matters a lot.

Sure, but coast-to-coast is not what matters. Moving people between where they need/want to go is what matters. I suppose a very large portion of all trips people in NJ makes on an annual basis is not coast-to-coast. It is within New Jersey or adjacent communities.