PASCO's old version of the cart is no longer available for purchase but we are still using the air tracks. So I created a 3D model that allows you to replace the metal ones without having to buy the new version of the trolley and having to change all the tracks.

This version of it doesn't have a plate at the front. Insted, I put a hole to place a magnet and on the other side it's possible to put a piece of velcro. This way, you can test collision between 2 cart on one side with 2 piece of magnet that repels eachother and the other side velcro, male on one of the cart and female on the other to make sure they stick together if they collide.

I added a bracket in the top for the photogate and you can attach a plate with the velcro if you want to test the collision with the magnet but use the sonar to track the movement.

The slot you see on the top are used to put weight on it and have a way to be equal overall the cart. The slot are made for round 50g and 100g weight.

For the licer, you'll need to place the back smooth (the hole for the magnet at the top) face down so that the surface where we want it to be the most smooth is not gonna print with support.
You'll need to have a minimum of 4 perimeters (I'm using 0.4mm extruder).
You also need to add a brim to minimize the warping.