EE WAS PASSENGER IN MANTRIP WHICH LOST POWER COMING UP THE MAIN SOUTH SLOPE. DRIVER STARTED PUMPING THE FOOT BRAKE BUT IT FAILED TO HOLD THE RIDE AND THEY STARTED ROLLING BACKWARD. DRIVER PUT IT IN PARK, WHICH DID NOT HELP. DRIVER THEN SET OFF THE PARKING BRAKE BUT THEY HAD HIT THE COAL RIB ABOUT THIS TIME, INJURING EE'S RIBS.