EMPLOYEE WENT TO PIT WELD ON HYD HAMMER ON EX 400 TRACKMACHING. HE WENT TO THE OPERATOR AND ASKED HIM TO MOVE MACHINE AWAY FROM WORK AREA. THE EMPLOYEE WALKED BACK TO THE WELDING TRUCK WHICH W AS 50FT AWAY. WHEN THE OPERATOR STARTED TO WALK THE MACHING A SLIVER OF METAL SHOT FROM THE TRACK AREA AND HIT EMPLOYEE IN THE LEFT FOREARM. EMPLOYEE HAD TO HAVE SURGERY AT HOSPITAL.