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.
From time to time I hang out at a forum dealing exclusively with Amtrak. The subject of bicycles and Amtrak rarely comes up (I'll keep to myself my opinion as to why). However, they respond to a wide range of questions with guesses, opinions, anecdotes, irrelevant comments, private jokes, and even some with what appears to be inside knowledge. But never the straight scoop like this.
JugglerDave, that's excellent. Before I moved to Salt Lake City, I was using Amtrak with my folder in Illinois. It was still a pain in the butt with many conductors and station agents often saying... "you need a bike ticket"
and then I would pull out that piece of paper... and I would get on the train with no further issues.