New Marx had two series of cars; one always had 8 wheels and is perfectly compatible as far as size with Marx 3/16 Scale. I am assuming you are talking about Marx Scale cars with the lower, scale-size trucks (Type B), not the larger 027-size ones (Type D). The New Marx may have tab-and-slot or fork-tilt couplers. The T&S will couple to the plastic Scale FT or the New Marx metal FT, but not original older metal ones. (The older ones lack a slot to accept the tab). Illustrated are the same Marx boxcar on Type B and Type D trucks.
American Flyer S are also compatible with the Marx Scale, although the cabooses are noticeably smaller than Marx. Illustrated are a Marx caboose with an AF on Marx Type B trucks. I have put a T&S coupler onto an AF car, but it is much easier to replace the AF truck with a complete Marx Scale truck. As the coupler is attached to the truck, coupler height is not a problem.
Marx plastic cars are somewhat larger than 3/16. The photo shows one of each for comparison. I do not think you would like to mix them, though Louis Marx Co. did supply Scale trucks on a small number of them. Hope this helps.