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 elements to create an action-packed course and bring the balls to the finish using magnetism, kinetics and gravity. With the GraviTrax ball track system you can discover gravity in a playful way and it can be expanded with various extensions for limitless building and playing fun! Add even more speed with the expansion sets.