The marble run was really cool, but I don't think it is applicable here. I also think you have the OP's track plan wrong. There's only one switch in the track, giving you three legs. Please correct me if I'm wrong, because everything I've said to this point is based on this track plan.
When the trolley gets to point C, you have to flip the switch the opposite direction. As I said earlier, you need to know where you came from. So when you reach A, you set the condition to B on the next throw. When you get to C, the switch throws routing you to B. When you get to B, you set the condition to A next, and arriving back at C completes the cycle. Repeat...
