'69' - can take balls!

I call this pendant simply - '69'

Partially inspired by my previous pendant Twin Rail Mobius, '69' is a set of four rails arranged in a figure 8 to make a train of balls travel in seemingly never ending 6 and 9 shapes.

Note: shell only is 3D printed, add your own 6mm or 1/4in balls!
If the balls stick, use just one ball to find the sticky spot and adjust the rails physically either with pliers or a screwdriver - doing so is at your own risk, so please blame no-one but yourself if injury is caused.