Every coaster has what is called an anti rollback device. This is what you hear when the train is going up the hill, the clack clack sound. If anything were to happen to the chain/cable the train would stop where it is. Now getting you out of that train and back on the ground is another story!
Have you seen the video of when the Top Thrill Dragster got stuck at the top of the crest? I still freak out when I see the guy on the lift getting out at over 400 feet in the air.