Well, you'll definitely miss the road trip experience...
but it does sound like you'll be roadtripping in the American West so you'll still get that experience then.
If you go on the Amtrak website into the "Routes" section, they have some nice interactive maps and a .pdf file that shows you all the major routes. You can get a good feel for what route you want to take and what areas it will go through. It's certainly not a roadtrip in the traditional sense but it can be a fun way to travel anyway. However, be aware that our train system is quite different than what you're used to in Europe. It's nowhere near as busy, nor does it go to as many places. From the .pdf file of the routes, it looks like they stop in lots of smaller towns/cities. If I were you, I think I'd do the route through Denver and Salt Lake City. But, dang, that northern route through Glacier NP also looks pretty darn cool. It's a tough call, all the cross-country routes will take you past some amazing scenery.
If this fits into your budget better, you might get a nice balance. I've never travelled much by train here in the US but the few trips I've done have been pretty cool. You see far different things than you do from the road because the routes are quite different....at least in my limited experience.
