It's just too bad that it's only convenient in a small number of cases. BOS->WAS (413 air miles) is certainly the best example we have in the US, especially since there's no non-stop service into National from Boston: you have to get into Dulles instead. Interestingly, the Acela Express on this route takes a little over 6:30 one way; a similar trip in Europe is Zurich->Berlin which takes just over 8 hours (with a change in Hanover) or London -> Lyon (471 air miles, 6:12 via Eurostar and TGV with a change in Lille/Europe).
Sacramento -> Los Angeles (373 air miles) is of course a joke at almost 15 hours (if you're lucky). Chicago -> St. Louis (258 air miles) is 5:30 ...
In a sense, it's even worse in most of the short-haul markets: Austin -> Dallas is 183 air miles, but takes 6 hours on Amtrak. Grayhound will do it in 3:05.
In the US, trains compete (poorly!) with busses.
/jordan