When i was into designing HO model kits, back when Tichy Train Group was Gould, my ideas for making a good running flatcar was to make the piece that flat of the flat car out of solid metal.
At that time i was thinking of etching. (chemical) Today i might use laser cutting.
Then the car would be at least minimally heavy enough.
Who makes good trucks for On30?
RE: Model Railroad Kit Review, an HOn3 flatcar kit