SUBJECT WAS TAKING A FLATCAR (SUPPLY CAR) OF BLOCKS TO 003 SECTION. WHEN GOING ALONG #4 BELT, IN BY HALFWAY POWER CENTER, THE MOTOR STARTED SLIDING. SUBJECT ATTEMPTED TO PUMP THE BRAKE, WHILE DROPPING SAND. THEN HE ATTEMPTED TO REGAIN CONTROL BY REVERSING THE MOTOR. HE COULD NOT REGAIN CONTROL SO HE JUMPED OFF THE MOTOR, RUNNING ALONGSIDE OF THE TRACK, STRIKING A TIMBER.