GraviTrax PRO - Starter-Set ExtremeTake your marble run to the edge with the GraviTrax PRO Extreme Starter Set! Test your creative limits and the laws of physics with the GraviTrax PRO Extreme Starter Set! Vertical components and four action accessories from the PRO series (Splitter, Mixer, Turntable, Helix) combine to create more challenges, more possibilities, and more extreme fun! Set ups a snap as children explore STEM concepts while engaging in independent, creative play. Simply