Making the same point, here's a news article:
https://bikeportland.org/2013/12/18/...arry-ons-98779
Again,
all trains in the Amtrak system accept folding bikes as carry-on luggage (in lieu of one bag).
In other words, Amtrak can't complain about your folding bike coming as a carry on. In theory, they
could claim that a rinko-packed bike is not a "true folding bike," although they don't have any reason to do so that I can see.