This Boston and Maine flatcar has a copper plated deck. I believe it is a late prewar piece. I have found several pieces from the late prewar period which have copperplate employed. Marx was probably using up existing stocks of material to produce as many train sets as possible before switching over to war production. The " copperplate" is not really copper plating, it is bright tin plate with an orange overvarnish.
(Courtesy of Stephen W. Jacobs)