The design makes perfect sense to me. All of the weight is on the pad, minus what the springs take away to keep enough pressure on the trucks to keep them tracking. It is different than the boxcar with a pad dangling under it.I've made plenty of those and the work fine, this just takes the weight on the pad to the extreme. The only weight on the trucks is what is provided by the springs, just like the spring on the pilot truck of a steam loco.
You can still accomplish the same with a boxcar/pad by attaching the weight to the rods inside the boxcar, instead of the boxcar itself. Note, however, that taking the weight on the pad to the extreme requires quite a bit of motive power to drag around...
