On Mon, May 16, 2022 at 10:11 AM, Bruce Smith wrote:

First, we’re really talking about cars with composite (steel and wood) or wood superstructures. While wood sheathing is applied to the interior of steel cars, they are not referred to as “double sheathed”, even if they really are. The sheathing consists of boards that are used to create the walls, floor, and sometimes roof, of the car.  On a single sheathed car, there is typically a single layer of boards that form both the inside lining and the outside “sheathing” of the car. These were typically attached to the inside of the metal superstructure (bracing) of the car, generating the modeling term “outside braced boxcar”.  On a double sheathed car, sheathing was attached to both the inside (liner) and outside (car body sheathing) of the car superstructure bracing. This provided greater protection from the weather for delicate loads that needed to be kept dry. In addition, it might be insulated, providing some level of temperature control.

Thus, on a practical level, single sheathed cars can be recognized by the visible exterior bracing, while double sheathed cars typically have a wood sided exterior with the bracing hidden behind the sheathing.

To expand a bit on Bruce's excellent explanation, sheathing is a generic term for the covering on a frame. When boxcar frames were made of wood, they were typically sheathed inside and out; inside to eliminate the nooks and crannies that loose lading, such as grain, could get into and later spoil, outside to protect the frame from the weather. The 25,000 USRA double sheathed boxcars original being discussed were some of the last, if not THE last, wood frame boxcars built.

With the move to steel framing during the years leading up to WWI, the question was raised, "Why cover the steel frame with wood, since the steel is more weatherproof than wood to begin with?" and it became the norm to only apply one layer of sheathing to the inside of the frame.

Dennis Storzek