I hope one day we can have one car that Amtrak can use on LD trains coast to coast. Right now the Viewliners are old and running out of use. The last few years a couple of trains (Three Rivers, Silver Palm) on the East Coast died and now the lack of diners is causing several LD trains to go diner-less or no diner at all (SS). Once the Viewliner II's are fully implemented the problems will hopefully be solved. But one day Superliners will have to be retired (or maybe the Viewliner II's do) and train travel will suffer on one half of the US while the other half will go unscathed. Having one train that can be used coast to coast will allow for more flexibility.
I know I enjoy being on the top level than the bottom level and prefer Superliners. I would love to ride them on the East Coast other than the CL. Also, I am guessing but may be wrong that a single Superliner coach has more seating capacity than a Viewliner coach. That might explain why the Western routes often have higher ridership than the East Coast. Imagine if the LSL or SM could use Superliners. How many more seats could they sell?
We all know Superliners can't go to NYP and I believe BAL is out (northbound and/or southbound)? Are there other areas where Superliners can't go? Are there any areas that require Superliners only? You can't do a Superliner from LAX-NYP but how about a Viewliner (assuming one day Amtrak has more than they know what to do with)?
A Superliner replacement which can be used anywhere is largely a solution in search of a problem. We already have a proven design which can be used anywhere; They're called low-level cars (Heritage, Amfleet, Viewliner, etc.). We also already have a bi-level concept which offers greater capacity. No car with low level boarding can really be used anywhere in the Norttheast Corridor as they require low platforms (Washington retains some); Superliners aren't prohibited just because of tunnel and other clearance issues. Talgo sets would also be unworkable. A bi-level car which could be used anywhere (Ie., use high or low platforms) would pay a very high price in terms of capacity, design, and passenger comfort and convenience.
You are perhaps familiar with the saying that "one size fits all" really means "one size fits none"? Multiple car fleets seem to offer sufficient flexibility. Certainly an all single-level passenger car roster could be used anywhere, but not all trains have the same requirements (you need high-speed equipment in the Northeast, but not on the
California Zephyr, for instance).