In the late 1950s, Marx sold a flatcar, labeled #5545 and lettered for the CB&Q (Chicago, Burlington & Quincy), with a large black clip in the middle. Marx shipped it with a pair of miniature trailers. These frequently got separated from the flatcar, so frequently you’ll find the car, sans vehicles, in the cheapie boxes at train stores and under the tables at train shows. The trailers are worth considerably more than the flatcar alone.
But there are some common, relatively inexpensive toys that work well on these common plastic freight cars.
From 1968 to 1984, Tonka had a line of “Tiny Tonka” trucks, about five inches long. (See this catalog, page 3.) The axles on the trucks happen to be just the right height to clip onto the Marx flatcar. Tonka produced a variety of these trucks, including dump trucks, fire trucks, cement trucks, and garbage trucks. So a variety of loads would be possible.
Marx also sold vehicles during that time frame that look eerily similar to the Tiny Tonkas–close enough in appearance that I suspect Marx copied them–so if you’d rather match up Marx products, the Marx trucks fit on the flatcar just as well.
Come to think of it, the size of the Tiny Tonkas and the competing Marx product lends itself well to the rest of the layout, too.