GraviTrax Expansion DipperDetails: Dip down from one level to the next! GraviTrax is a unique marble run system featuring hexagonal risers and track tiles that simply fit and stack together, rail tracks that hook into place, and stunt pieces that add exhilarating action. This set requires the GraviTrax Starter Set. One marble acts as a weight while the bucket waits at the top for another marble to come rolling in. Once it does, it swings down, drops it onto the lower track,