The Northeast Corridor has at least hourly service through out the day, 'high-speed' (for america) trains, and lots of newish comfortable equipment, it's arguably the most civil way to get between NYC/DC/Boston.
A lot of the rest of the lines have once-per-day service, that runs on old equipment along freight lines. If you want to ride the Empire Builder from Fargo to Minneapolis, your only option is to get up at 2 o'clock in the morning to catch the one train of the day, and if that train had to wait for a freight train (which has priority) along it's previous 2 day journey from Seattle, it wouldn't be unusual for it be delayed until 3 or 5am.
Since the US doesn't invest in passenger rail outside the NEC, it becomes less and less viable for anyone to actually use it.