For anyone who's interested, two years ago today Amtrak revised its conductor rulebook ("Operations Standard") to formalize allowing Folding Bikes, which previously had been mentioned as allowed on the website but in practice prohibited by many conductors.

Here is the PDF, be sure to carry a printout with you just in case!
http://www.transalt.org/files/resour..._policy_07.pdf

I'm personally at 295 round-trip commutes with my Downtube folder on Amtrak over the past two years!