The key determinant that tells you whether trains or planes are more cost-efficient is population density. If you have two cities far apart with not much in-between, then it's cheaper to build a couple of airports. If you have loads of other cities along the route, then it'd be cheaper to build a railway. The former case is true for much of North America, and the latter for much of Western Europe and UK.
Commonly cited but not really grounded in reality— lots of places that have made rail work have very similar or worse density characteristics to the coastal US routes that should be a no-brainer:
