One more tidbit: I've now read the July 5, 1896 (p. 10) Chicago Tribune article mentioned in a previous post. It offers a wealth of information on sandwich wagons, their menu and their patrons. The article's too long to quote in its entirety, so I'll just post an excerpt here:
"The piece de resistance is chicken sandwich, which consists of a quarter section of a small spring chicken, cold, placed between two slices of bread, with the accompaniment of a pickle, a green onion, or a dash of catsup. This costs 10 or 15 cents. Pork chops, ham, pig's feet (fresh or pickled), fried fish, codfish cakes, and eggs are also put in sandwich form, mostly at five cents each. A distinguished favorite, only five cents, is Hamburger steak sandwich, the meat for which is kept ready in small patties and 'cooked while you wait' on the gasoline ranges. Fried oysters, breaded, are also a popular sandwhich ingredient.
The sandwich man takes in from $5 to $7 a night, perhaps $12 on Saturday or other special nights -- this 'special' implying when many people are out late -- and makes an average profit of 25 percent on his outlay, say $12 a week, besides much of the food required for himself and family. Many of them keep no horse, but hire one at 50 cents a day to haul the sandwich wagon to and from its nightly stand. It is a business principle of the class to sell out everything, if necessary even at startling reduction, before going home in the morning, so that supplies for next night may be all fresh cooked."
The Trib article includes an engraving of a sandwich wagon , showing the proprietor and two patrons. It appears to be smaller and much less elaborate than the wagons produced by the Worcester company, and it seems that there's no room for seating inside.