The easiest way to offer these would actually be as 2-car add-ons. Here's my rationale:
--There are no solid Viewliner trains: And there might never be. They are always mixed with Amfleet cars, which are a long way from retirement. There's not even an agreed-upon design out for their replacements. They may look like Viewliners, they may not. (chief problem with a Viewliner-style coach is the upper windows would compromise overhead luggage space. Maybe a Viewliner shell with a single row of Acela-sized windows. But nobody knows yet)
--More affordable as a two-car add-on: You can offer them as sleeper/diner and sleeper/sleeper sets at the outset. Currently there is only one Viewliner diner, the 8400. The train that has that particular car will have at least a couple of Viewliner sleepers. Others will have Viewliner sleepers and a Heritage diner till the V-II diners are finished, giving folks the option of adding sleepers to current Amfleet sets to re-create trains that have the 8400 or trains that don't (until the V-II diners are delivered).
--Allows tooling costs to be spread out a little: Baggage and Baggage/Dorm cars can come out later. And even then, a baggage/baggage-dorm set can be added on to existing Amfleet sets.
---PCJ
