I have several acquaintances who are amtrak employees, 1 engineers and a couple conducters. In speaking with them, amtraks policy is a clear "no tandems allowed" however it is ultimately at the discretion of the conductor running that particular route. They did all concur however that a coupled tandem that was split prior to boarding and could hang from the standard bike racks in the cars, with both captain and stoker having bike reservations, could not be turned away. This idea was recently successfully tested on the pacific surfliner out of san diego.