Actually,USRA single sheath boxcars had bottom-supported doors. I think the extension of the upper door track to the left of the door was to strenghten the top chord of the side truss at the door opening,just as the top chord of many hopper cars were reinforced in the middle area of the car.Although the hopper cars often had this added after their original construction,the USRA box cars had this reinforcement when built.Probably the presence of an opening in the side made the need more obvious.
