^^^Europe's cities are much much closer together than cities typically are in America (outside of the NE where Acela and other trains/popular busses are). I don't think Europe and America can be compared as apples to apples, outside of the NE, CA (and even here it's really all about SF/Sac and LA/SD and there is a large gap between), and upper Midwest.
Also, trains much cheaper than cars? Maybe in parts of Europe that I haven't been, but popular train routes/corridors in the US (and Amtrak in general) is more expensive than it would be to travel by car if you are by yourself, and especially if you're more than 1 person (as opposed to 2-4+ people in a single car). I think the argument here is not price, but rather efficiency, time to travel, ease, etc. I mean, you are paying for point to point convenience and smooth, comfortable travel (trains are by far the most "comfortable" way for the masses to travel).
Acela drops you off from the center of one city in the center of another, and navigating train stations is much faster than airports.