A 'smaller than 34" x 15" x 48" (860 x 380 x 1120 mm)' bike is just another piece of carry-on luggage. What would they do if the eight of you had your folding bikes stowed and subsequently a theater troop boarded with their packed sets, costumes and props? Too much carry-on luggage on every car on the train? Hmph. Wonder if that's ever happened in Amtrak's 52 years?
Yeah, and add a couple of guys with tubas and bass viols. My guess is that the first person to try to board after all the storage space is filled up is out of luck--the musical chairs principle, so to say. But that's just a guess. And I don't know how often that happens, if it ever does.