Yee haw! That looks like a lot of fun. I bet I could build something like that if it weren't for all the physics involved.
I have some land just doing nothing right now and I've thought about something to put on it. A rollercoaster never entered my mind, but this guy made me think. I also think I'd have the same problem as you, the physics involved would mean that the car went off the track a few times before I got it right.