1. IN A RAILROAD FLAT CAR HAVING A WHEELED UNDERSTRUCTURE INCLUDING AN ELONGATED FRAME SUPPORTING A RECTANGULAR, SUBSTANTIALLY HORIZONTAL FLOOR, THE COMBINATION OF BRACING FRAMES EXTENDING TRANSVERSELY ACROSS SAID CAR AT OPPOSITE ENDS THEREOF, EACH OF SAID BRACING FRAMES COMPRISING A PLURALITY OF L-SHAPED MEMBERS DISPOSED SIDE BY SIDE IN SPACED RELATION AND EXTENDING TRANSVERSELY ACROSS SAID CAR WITH EACH OF SAID MEMBERS HAVING HORIZONTAL BASE PORTIONS EXTENDING ALONG THE FLOOR AND INTEGRAL VERTICAL DISPOSED LEG PORTIONS UPSTANDING FROM SAID FLOOR, A HORIZONTAL RAIL EXTENDING TRANSVERSELY ACROSS THE CAR AND CONNECTING THE UPPER ENDS OF SAID LEG PORTIONS OF EACH OF SAID BRACING FRAMES, SIDE GATES DISPOSED ALONG OPPOSITE LONGITUDINAL SIDES OF SAID CAR AND PROJECTING UPWARDLY FROM SAID FLOOR, EACH OF SAID SIDE GATES COMPRISING A PLURALITY OF VERTICALLY DISPOSED STAKE MEMBERS SPACED APART LONGITUDINALLY ALONG SAID CAR, MEANS MOUNTING THE LOWERMOST ENDS OF EACH OF SAID STAKE MEMBERS TO SAID FRAME, A HORIZONTAL RAIL EXTENDING LONGITUDINALLY OF SAID CAR AT EACH SIDE THEREOF AND BEING SECURED TO UPPER ENDS OF THE STAKE MEMBERS, MEANS DETACHEDLY CONNECTING SAID TRANSVERSELY DISPOSED RAILS TO SAID LONGITUDINALLY DISPOSED RAILS AT THE CORNERS OF SAID CAR THEREBY TO FORM AN INTEGRAL STRUCTURE HAVING TRANSVERSELY AND LONGITUDINALLY OPPOSED VERTICAL WALLS FOR RECEIVING THEREBETWEEN A STACK OF SHEETS LAID HORIZONTALLY, ONE UPON THE OTHER, AND FOR PREVENTING LONGITUDINAL AND LATERAL SHIFTING OF THE SHEETS RELATIVE TO SAID FLOOR, RAILS DISPOSED TRANSVERSELY OF SAID CAR INTERMEDIATE SAID BRACING FRAMES AND SPANNING SAID LONGITUDINALLY DISPOSED RAILS WITH THEIR OPPOSITE ENDS REMOVABLY ATTACHED TO SAID LONGITUDINAL RAILS, AND CUSHIONING MEANS DISPOSED INWARDLY OF SAID BRACING FRAME TO CONNECT THE END EDGES OF THE STACKED SHEETS.