GraviTrax Gravity Hammer $12.99 Default Title - $12.99 Quantity This product is currently sold out. Please fill in the form below if you'd like to be notified when it becomes available! Propel the marble through your run by adding the Hammer. When triggered, the hammer swings down and hits the marble, giving it momentum to send it on its way. This accessory also includes 6 additional tracks. GraviTrax Starter-Set required. Recommended for children ages 8-99.