Yes, Jim.....you are correct about the blue sand car being Plastimarx.
However, seeing that it has a lineage from Wyandotte to Hafner, to Marx, to Plastimarx , allows it to be considered in the makings of a very colorful tin train set. Now if I could only find the FNP red sandcar. BTW, I live 30 minutes from the location of the old Louis Marx Toy Co. in Girard, Pa. I probably had Marx toys as a young kid too !