If you're interested in the history of why Europe has such a dense train network, and the United States doesn't, it's partially about war.

There was a multi-part show I saw on BBC Four about a year ago that explained that Europe's train network isn't the result of some kind of visionary urban planning or anything altruistic. It's because European nations have such a long history of fighting one another. And in the early part of the last century, the continent went on a rail-building frenzy because of WWI.

The United States have only gone to beating each other up once, and that was before trains were viable as large-scale tools of war. Perhaps if the Civil War was delayed by a couple of decades, things might be different.

Of course, there are many other factors, but this is one that I find interesting and almost never explored.