EMPLOYEE DOING END OF SHIFT INSPECTION AND FUELING EQUIPMENT IN PIT AREA. MACHINE WAS PARKED NEXT TO FUEL TANK. UPON COMPLETION OF FUELING, HE ATTEMPTED TO STEP DOWN BACKWARDS OFF FRONT END OF MACHINE TRACKS. DUE TO WET, MUDDY CONDITIONS OF TRACKS, EMPLOYEE SLIPPED CAUSING HIM TO FALL,BUMPED HEAD ON BUCKET OF MACHINE.