The MK Viewliner shells were made at the former Pullman plant in Chicago. There is a lovely picture of one, albeit reversed, on page 29 of the Sept. 2009 issue of Trains.
i suspect the main reason for choosing stainless is to reduce the amount of reverse engineering required .
with modern metal treatment and paints i don't believe stainless has that much of an advantage over other materials . it certainly has some disadvantages , probably the main one being its simply a bitch to work with .
Dare i say the bare metal look of amtrak's cars looks old fashioned these days , what would really stand out as new , would be a nice classy paintjob .