Is it? Part of the cost is the fact you're going to be behind the wheel for hours at a stretch, plus you have to make pit stops while the train keeps running while you eat and sleep (and I speak from experience; I once rode the California Zephyr from Chicago to Oakland). How about some numbers to show just how much of a premium you're talking.
That said, most would say unless you're willing to pay for the slower time or you're hauling (in which case you're probably driving a truck in any event), you're probably better off flying there if you have to go more than halfway across the country. Not as expensive as the train, and you get there in a few hours at most.