The Northeast Corridor metro areas (Boston-NYC-Philadelphia-DC; dense, 4mm+ pop centers all located within a few hours of one another) and a handful of other regional short-haul routes (San Diego-LA, Boston-Portland, etc) account for almost all Amtrak traffic; the long-haul cross-country routes are basically tourist attractions subsidized by the commuter rail segments of the system.

http://www.amtrak.com/servlet/ContentServer?c=Page&pagename=...