The problem is that there's so little vertical room between the bottom of the frame plate and the railhead, plus there are axles and mounting screws in the way. This photo shows one of the options I came up with (I know, it's really far forward). I also mounted a vertical sprung roller to the trailing truck. It might have worked, but it just derailed the truck all the time, and there's no room on it to add weight. Adriatic, it sounds like you might have been able to rig something up, but it just got beyond my abilities!
