I think there is some confusion between "cafeteria" and "buffet" cars here, though I'm not sure that Amtrak themselves really had a standard for what some of these words really meant (and they didn't used to write them on the sides of the cars themselves). Amtrak used to call some of the buffet cars "buffet - diner - lounge" cars, so even they seemed a little confused. I remember the cafeteria cars being a precursor to the modern "cafe" cars, where you could buy snacks and light meals. The "buffet" cars were what a few people have described here so far, more of a real dining car but with a buffet setup, and these may or may not have been part of the earlier tandem kitchen/diner set. I know at least a few were converted lounges and/or observation cars, so they clearly weren't *all* originally a tandem set.
I never rode in a buffet car that I remember, but I do remember the cafeteria cars at least somewhat clearly. I remember them being basically a heritage lounge car outfitted with cheap looking tables and spartan seating (not booths as I recall) and then a basic snack counter like in an Amfleet cafe car, but it wasn't set up exactly like that. I remember it being more like the bottom level of a Superliner lounge car, somehow. I think the snack bar section may have been in a corner of the car, similar to the bottom level of one of the Great Domes (like this, but without the decoration or the booth seating:
http://www.gngoat.org/domes09.jpg)
My memory could be a little off, as I was pretty young, but that's how I remember them.
Also, in 1981-82 the Crescent, at least, would have still had a full bar car, which was distinct from the regular cafe/lounge or buffet/diner car. This was not regularly advertised, from what I remember, maybe because they didn't have that many cars or they just didn't run them every day. But I can remember riding the Amtrak Crescent around that time and being absolutely amazed to walk through the train and come upon a bar car in its original decor, complete with piano and everything. So I think they may have used the "cafeteria lounge" nomenclature in part to distinguish from their "real" lounge cars that they still had at the time. A cafeteria lounge was a place to buy food and eat it; a plain old lounge was just a place to relax.
In those days, though, there was still a lot of variation in Amtrak's equipment, even among cars called the same thing. They made an attempt at standardized naming but a lot of these cars they had were highly customized for the railroads they came from, and they never bothered to update a lot of them. So you ended up with confusing things like a "buffet - diner - lounge", a "cafeteria lounge" and a plain old "lounge" all running on the same train and all configured differently. They would just add words as needed to describe a car's function, but even two regular "lounges" could look totally different inside.
I suppose that means it's also possible that there were different types of "cafeteria" cars too.
Edit: Whoa, I was mostly right!
http://library.duke.edu/digitalcollections...ess.T2486/pg.1/
It's possible Amtrak did serve hot food out of these for a time, I guess, but when I rode on them, they were just using them as a snack car. They're definitely not the same as the buffet diners.