EMPLOYEE WAS WORKING ON 5A - SECTION. HE WAS IN PROCESS OF TRANSFERRING 4 INCH I BEAMS X 10 FEET LONG FROM SUPPLY FLAT LOCATED IN TRACK ENTRY ONTO PETTITO MULE. AS HE LIFTED BEAM, HE FELT PAIN IN LEFT SHOULDER - POSSIBLE TORN ROTATOR CUFF. WAS INFORMED OF DIAGNOSIS ON 1-23-04.