Just browsing thru some Amtrak pages. Indeed they quite bluntly say "Motorized bicycles are prohibited." The customer could try to interpret "motorized" as gas motor only (which would be logical because of fire hazard) and try his luck checking an ebike in. I doubt no one will even notice some of the stealthiest bikes especially if battery is taken out. Funnily enough (at least for a nonamerican) Amtrak goes to great lengths to allow carriage of firearms. Real western style. Ok, I dont think assault rifles or MGs were allowed.