Did a little googling yesterday and came up with some great info on sandwich wagons/buffet cars. One internet site excerpts parts of a Chicago Tribune article from July 5, 1896 that describes lunch wagons. I wont quote it here, but I'll provide the link below. Apparently, a company in Worcester, MA mass-produced lunch wagons, and by the late 1890s they were present in urban areas across the nation. According to this article, the wagons provided seating for patrons (!) and eventually they evolved into the classic (stationary) "diner".
Found the site of the E.B. Luce Corporation, a long-lived photographic company, whose archive includes old shots of lunch wagons produced by the Worcester company. RST, I doubt the Chicago Historical Society has any of these, but the photos on this site are wonderful. I also doubt that all wagons were as elaborate as these.
The URL for the EB Luce archive is: