[chiph]

You have more room, mostly. A Prevost bus conversion is 45 feet long while a railcar is 80-ish feet long (less a few feet for the vestibule and/or rear platform). The rail car is wider, except when you park your bus and put the slide-outs out.

Of course, travel options are significantly broader with an RV since paved roads go everywhere. However you can't take an RV into some cities in the northeast - I know that Washington DC and Baltimore have banned them from their streets. Whereas Amtrak takes you right into the city center.