Ravensburger's interactive ball track system. With the GraviTrax ball track system, you can build your own ball track world by creatively using the laws of gravity. Use the various building blocks to create an action-packed course and guide the balls to the finish line using magnetism, kinetics, and gravity. With the GraviTrax ball track system, you can playfully explore gravity, and with various expansions, it can be expanded for endless building and play fun! Add even more speed with the expansion sets.