From the Amtrak website:
Only true folding bicycles (bicycles specifically designed to fold up into a compact assembly) are acceptable. Generally, these bikes have frame latches allowing the frame to be collapsed, and small wheels. Regular bikes of any size, with or without wheels, are not considered folding bikes, and may not be stored as folding bikes aboard trains.
I said upthread I didn't know why Amtrak cares how your bike gets into their 34" x 15" x 48" (860 x 380 x 1120 mm) "folding bike for carry-on" envelope. Seems arbitrary. But based on what Amtrak's website says, I can not assure cycletourists that their coupler or rinko'ed bike will be accepted as a carry-on by each and every one of Amtrak's conductors.
I think you are correct about Amtrak not caring how a particular bike gets small. They shouldn't care, and as far as my experience goes, they don't care. The key wording in the Amtrak quote about folding bikes is
"Generally, these bikes have frame latches allowing the frame to be collapsed, and smaller wheels." What they mean by "generally speaking" is that latches and small wheels are typical of folding bikes, but--as that clearly implies--there are exceptions. My standard frame, with its full-sized wheels, rinko'd and packed in a bag that falls withing the size and weight limits, is one of those exceptions That's my position, and I am sticking to it.. My advice is not to overthink this.