Well, already produced, there is the helium car, the jumbo whale belly tank car, the open frame toxic chemical container car, and the scale weight car that I can think of right off hand. And then there are the BN experiment Box Car/Hoppers that never really worked as planned. And Santa Fe built an articulated five unit grain hopper.
For unusual cars there are the earlier woodchip cars. GN, NP, and SP&S took some early double sheathed wood boxcars took the tops off and braced the interior and corners, sealed the doors, and added about two foot extensions to the top and ran them in woodchip service for a number years before being replaced by cars purpose built for that commodity. And the even earlier hog feul cars which were essentially flats with sides and small doors along the side for hauling woodchips to power plants.
American Model Builders have a new Laserkit to convert a 40 or 42 foot flat car into a pickle tank car...unfortunately 40 and 42 foot flats are hard to come by (Red Caboose, now owned by Intermountain does make a 42 foot flat, they're just kind of hard to find at times)
