Well, sure, there are similar stairs on the MARC bi-levels (or tri-levels, I guess), but those are commuter cars, and a lot of revenue space will have to be sacrificed for the stairs.The design of European and Japanese double trains feature easy up and down stairs at entrance points making boarding with luggage far easier.
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I'm starting to find it a little hard to haul up luggage to the upper floors, and I quite often use the luggage racks on the lower level. And people with real disabilities have the Handicapped room on the lower level, or even a lower level roomette, so there really isn't a need to have accessible stairs on the Superliners.