Rick; When I first went to work for the RR we were
still repairing a few cars with the old style roof with the individual roof sheets that met at the center and that were secured with seam caps between sheets and down the center of the car. The design was intended to make them weather
proof so as to avoid damage to the lading from blowing snow, water leaks and intrusion of dirt and cinders. The fire resistance was important but it was just part of the package.
The present design of steel roof is strong, light
weight and quickly assembeled. The individual sheets cover the full width of the car and are fastened together with a roof rib that also provides structural strength. On new car programs we built the roof on a ground level jig and then set it on the car body.In box car programs we usually ran 14 cars per 8 hr day.
Jim