A very quick search for Amtrak car #3954 says this car was originally NYC #3035, a post-war lightweight. In 1969, it was reconfigured to a Penn Central 50-seat snack bar coach. In 1971, it was sold to Amtrak as #3954, and then in 1982, it was bought by the owner of the diner. Source: 12 November 2014 reply "Part Two: Postwar Lightweights, Continued:" to post "Surviving NYC Lightweight Cars; Railway Preservation News Post.
Stairway added to small room for private parties mentioned in the take-out menu (?):
Some embellishments were added for sure ...
Notice how the under carriage is original: