Ravensburger's interactive ball track system. The Accessory Ball Box is a must-have for every GraviTraxer! The four boxes, each featuring the cool GraviTrax height markers, are green, gray, and white. Two of the boxes are filled with silver and colorful balls, allowing you to add even more to your track. With the GraviTrax ball track system, you can playfully explore gravity, and it can be expanded with various add-ons for endless building and play fun! Add even more speed with the expansion sets.