A runaway train is a type of railroad incident in which unattended rolling stock is accidentally allowed to roll onto the main line, a moving train loses enough braking power to be unable to stop in safety, or a train operates at unsafe speeds due to loss of operator control. If the uncontrolled rolling stock derails or hits another train, it will result in a train wreck. A parked train or cut of cars may also run away if not properly tied down with a sufficient number of hand brakes.