I'm not sure how you're making it (I haven't checked, and can't right now), but I would make the train/cart a func_tracktrain, and make a prop_dynamic with the tracktrain as the parent. Then, create a trigger_multiple with the parent of the tracktrain, and have OnStartTouch and OnEndTouch Enable/Disable the tracktrain. Not sure if it works, but it's a theory.