The fact that trains are far more economical than airplanes or buses on relatively short distances over relatively densely populated areas (such as Northeastern states, Chicago environs, and San-Francisco, Los Angeles, San Diego corridor) - but cannot compete with planes on longer distances (e.g. coast to coast or even NYC - Chicago).
Of these areas, only Northeastern states have something that resembles functioning rail transit system - albeit limited mainly to the NE corridor, but things start rapidly deteriorating when you move West.
What is more, the travel across NE corridor is prohibitively expensive. The round trip between DC and NYC is $144 on regular Amtrak and $314 on Acela express. For a comparison, round trip between Lisbon and Porto (comparable distance) on the 1st class Alfa Pendular (comparable to Acela express) is about $50.
What makes the difference is the level of government subsidies - but in this country subsidies are used to make less efficient modes of transportation in this particular market niche (cars and planes) more attractive vis a vis trains. For example, the departing governor Glendening gave huge subsidy to airline industry to provide affordable transportation for small cities in western part of MD. It would make better sense to improve the existing tracks to provide fast train connections, but evidently political pork prevailed.
It would be fair to say that US is socialism for cars, welfare state for corporations, and dog-eat-dog capitalism for everyone else.
Wojtek