The 967 NYC side dump car is one of the more difficult Marx plastic cars to find. It makes you wonder, why, in the four wheel plastic line, which targeted low end sales, would they make a car that required two distinct molded exclusive parts? I guess that is why they are hard to find. They didn't put them in many sets.
The black car is the harder to find. It came only in an early 4 wheel plastic set headed up by a numbered 994. The blue car came in a KCS set and also in a set headed up by the 994.