The appearance of some blocks are different from their physical shapes. It can be seen when two blocks are in contact. Here is the result of some investigations and discussion with other members (including Martin Magni). The real physical shape of the collider is in blue. For the curved rails, you can look at it as if the ground was at the bottom of the drawing, or as if you looked from under.
BTW the mechanism makes the movable parts being composed of more than one block very liable to get stuck.
