I also looked at prototype photos I've taken of wheel cars. I noticed they don't really look like the loads for sale on eBay. For instance, here's one:
As opposed to the eBay loads, the prototype often has wheels nested between each other in alternate rows, and there's an outside bar on which they rest.So I gave this some thought. What I did was lay out a piece of wax paper, and then I nested wheelsets in alternate rows, up to a length short of 50 feet. I cemented them together with CA glue. I did it against a steel ruler to keep the string straight. Then when the string was dry, I added some Evergreen styrene strips that I had on hand, two to the outsides and two to the insides behind the inner wheels in the string. Then I added short end pices.
I cleaned off any accumulated CA from the assembly with acetone applied with a cotton swab. Then I sprayed the assembly with Krylon black from a rattle can, followed by Testors Rust enamel from my airbrush, thinned with acetone. Here's the result, mounted on a Walthers Trainline D&RGW work flat:
This took part of a morning and part of an afternoon and cost almost nothing.