I only really have experience in the eastern half of the U.S., but on some routes I've found Amtrak significantly cheaper (in coach) than flying, so I consider

I only really have experience in the eastern half of the U.S., but on some routes I've found Amtrak significantly cheaper (in coach) than flying, so I consider it if I'm not in a hurry and it's a day train under ~8 hours (not sure if that qualifies as "modestly long"!). The price difference is especially large in Amtrak's favor if you're doing a one-way trip, or to/from smaller cities that don't have much airline competition. For example, to pick Richmond, VA as a city I know about. Amtrak's Richmond to NYC service is about 6 hours 15 mins, and as low as $32 each way if you buy in advance. Flying is only around 1.5 hours but typically $150 one-way or $200 round-trip. Going the other way, Richmond to Charleston, SC is typically $68 one-way and 7 hours, 15 mins, versus about 3.5 hours and $200 to fly one-way.