Created a EP Curve in the shape I wanted and then used a poly cube and did an extrude from one face, then increased the divisions to 50. set the track as a passive rigid body and the cart to an active rigid body. Added Gravity to the cart and adjusted the Center or Mass then finally put initial velocity Z to 1.

I did say theres a lot of tweaking to be done to get ths right, but dynamics will work bettter then motion paths. good luck