INJURED HAD MADE REPAIRS TO A 50 TON LOCOMOTIVE AND WAS USING AN OVERHEAD CRANE TO REPLACE A COVER PLATE OVER THE BACK TRUCKS. HE WAS LOWERING THE COVER AND GUIDING IT WITH HIS LEFT HAND. THE CRANE HOOK CAME OUT OF THE HOOK-UP ALLOWING THE PLATE TO FALL AND PINCH HIS LEFT RING FINGER BETWEEN COVER AND TOP OF MOTOR.