Quote Originally Posted by curious aardvark View Post
And on screws by the look of it.
Not quite, they are straight-cut gears (think of a spline shaft), just very long. The bottom of the bed contains a rack making up one half of the rack and pinion. If you only spin one of the two shafts, it the opposite direction's rack will just slide along the gear with no translational movement. It is a great idea - it makes both guidance and locomotion all in one simple system with very few moving parts.