as a frequent Amtrak user (NY to Boston) I've pretty much given up on carrying the bike- even the folder.
The Amtrak employees seem either clueless, intentionally vague or at times downright hostile when approached about bringing a bike of any kind on any of the trains.
The Acela can be real tight for baggage of any kind and during peak times even a folder can be refused.
The best bet is a good bag for the folder and not let them know it's a bike- it's kind of like gays in the military- don't ask, don't tell.