While you can track down 21 different lettered Marx 3/16 cars (maybe 22 if there is more than one NKP dump car),
that is it, although there is "deluxe plastic" with tilt fork couplers to run with it. There seem to have been
fewer hopper cars, stock cars, and flat cars made in the series. The hopper cars do look more expensive to manufacture, with working hopper doors, etc., but not so much the stock cars with another punching action, or the flat cars with railings riveted to the floor (on most versions). I had hoped when these were redone, they would be identical to the Marx selection, EXCEPT, of course, new and different roadnames, and hopefully, new stampings for the hopper, shortening it, making 2 and 3 hopper versions. Box cars could be extended slightly as double door versions, gondolas extended as mill gons, tank cars rendered as double dome versions, and cabooses lengthened slightly into off center cupola varieties. These as all additions to the Marx-type stampings.
That did not and probably will never happen. Every now and then you will see someone's customization of one
of the original cars. I did it when I was a kid.
Just musings over what might have been.....