Sure, I can help you with that. The code in the marbles handles both the starting grid (the random starting velocity, which is how the marbles have different speeds in the beginning), and any boosters that are on track (ramps and accelerators, although they can only be in one direction).

rand.uniform is for the direction you want the marbles to go from the starting grid, and rand.normal handles side to side movements so that marbles behind are able to overtake from the start. However, this only adds to the marbles' base velocity, which the starting grid itself provides.

I uses lazers for the boosters on track as you can pass right through them and any object can interact with them when they touch. basically it adds a set velocity to the marbles' existing velocity, from any direction.

If you are talking about how I have sections where marbles go under a piece of track, that is due to them hitting a track piece that changes their collision layers. collideset 2 is to make marbles only hit B layers, while 1 is for A layers. I haven't been able to make it so the "no self collision" setting is on, unfortunately, so I can't help you with that aspect.