The bottom line is neither Amtrak nor the USPS will ever be profitable because Congress will not allow them to be. Amtrak was profitable for a couple of years when they were running mixed trains but the private sector whined to Congress so they were told to stop. My favorite Amtrak trip was and always will be on the 3 Rivers from Chicago to Philadelphia. 33 cars long (limited by the railroad) but only 4 of them carried passengers. Shippers loved it.
And so long as the USPS has to continue rural delivery they won't be profitable either.