INJURED WAS CLEANING A GONDOLA RAIL CAR USING A WALDON FRONT END LOADER; THE BUCKET OF LOADER CAUGHT ON THE END OF THE RAIL CAR. THE BACK OF THE LOADER LIFTED OFF THE GROUND THEN SLIPPED OFF. WHEN IT JARRED TO THE FLOOR, IT SPRAINED INJURED'S BACK. INJURED HAS HISTORY OF BACK PROBLEMS. COMPENSATION WOULD NOT RELATE THIS TOA PREVIOUS INJURY.