A 22uH choke in series with the center rail pickup is the solution. Obviously, you have to open them up to do the mod. For many steamers, you'll have to do the mod on both the locomotive and the tender. However, you can leave just one on the track and see which one is the problem piece and modify that one.
Some lighted passenger cars are also a problem, anything that steps on RF signals between the center rail and outside rails will affect the DCS signal.
