As we in American emerged from the austere 1930's , Marx observed Lionel and American Flyer's success with "scale trains" Marx felt he had to compete and started to develop a scale train line in 1940,41 and early 42. It was ready at Christmas 1942 and the first marketing of this line appeared in the Spiegel and Ward's catalogs for 1942. These were an interesting compromise. They were "S" scale at 3/16" to the foot of the prototype however, Marx did not want to re-tool all his other trains and track so they remained 0 gauge. I am told, in their day, these rather handsome lithographed cars attracted American Flyer fans enough so that they changed the trucks and couplers to run then with the rest of their Flyer trains. That is just hear-say of course as I cannot confirm this. The cars were all metal and lithographed with excellent detail. Of course the US Government suspended any metal toys from manufacture after 1942 to conserve metal and support the war so these cars didn't really enter the market place until 1945 and they continued until 1957.
The cars used what came to be called "low" scale freight trucks which can be observed here. There was some very limited use in the late 50's and early 60's of some of the cars using the high, more common Marx toy train trucks.
Best Wishes, Don