It's usually very reasonable along a rail route.
Check out the prices for when you have to change trains, like to go from DC to Chicago. I can fly first class for that!
I went from Syracuse to Annandale VA (just south of DC) for something on the very close order of $120, with a train change in NYC (Penn Station).
Biggest problems IMO with train travel, is that if you need to go further than 600 miles or so, you're likely looking at a multiday trip, and that the rails don't go NEARLY everywhere one might want to go. I can't spend more than a week just getting from the DC area to Los Angeles and back for a vacation, and the roomettes or family "bedrooms" range from $400-$1200 over and above the price of the tickets per leg (DC-Chicago, Chicago-LA, LA-Chicago, Chicago-DC - that's 4 legs). Pretty pricy - yeah, the airplane is even more cramped, but you're not on it NEARLY as long.