After carefully studying the prototype photos, I noticed that there was a bit of a rounded edge to the side frames of the car body. I have found that to be a distinctive feature, because other flat cars don't have that. I used an emery board to gently file this curve to the top and bottom edges of the side frames.
I installed the Grandt Line major under-body brake components. I decided to not model the pipelines, because they are just not visible during normal operations. There are only so many hours in a day!
OK, so now for the actual stake pockets construction. If you model the flat car as used for carrying LCL containers, you don't need to model the stake pockets. However, all three of mine are going to be used for carrying other loads. There are twelve stake pockets on each side of the car. The photo below shows pencil marks identifying the center location of each stake pocket. I start by making a mark 5 feet from each car body end, because there appears to be one directly in line with the truck bolters' center. I then marked the outside one just inside (toward the car center) of the truck's roller bearing housing (upon later research I found that it should be centered above the outside roller bearing housing). The third stake pocket needs to be just toward the inside (car-center) of the inside roller bearing housing. Once those three marks are made on each end of the car, I take an exact measurement of the total distance between the inner-most marks and divide that distance by 7 to give me the equal distance between the middle set of 6 stake pockets (which came out to just under 41 scale inches).
I looked online for S-scale stake pockets and studied prototype photos, but in the end decided to just make my own (yes, I scratch-built all 72 of them). They are not an accurate model of the PRR FM stake pockets, but they come close to their overall shape. To start, I cut an equal length of one scale 1x6 strip and two scale 1x4 strips. The 1x4 strips are glued on top of the 1x6 on their long edge, to form a deep C-channel (I was not able to find a pre-made C-channel styrene strip that was of that size). This is a delicate operation, so what I do is put the 1x6 up against the metal square's base (which is perfectly flat), and whose other end is held such that the metal base and the glass surface I work on are at a 90-degree angle to each other. With the 1x6 flat against the metal base, I then carefully position the 1x4 against the edge of the base and apply Testors glue to the joint. I let this dry for a good 30 minutes or so. The styrene strip will stick to the metal base, but using a razor blade, I can carefully free it from the metal. I then flip the half-C-channel over and glue the other strip in like manner, again giving it lots of drying time.
I set up my NWSL Chopper II to cut a scale two-foot section of this fabricated C-channel to be the stake pocket body.
To glue these onto the side frame, I lightly clamp the flat car body in a vise. The best way I found to ensure that the stake pockets are centered over the pencil marks and vertically straight on the body was to put a dab of Testors glue right on the pencil mark, making sure the dab is about as wide as the width of the stake pocket. Then, using tweezers, I placed the stake pocket on the glue, positioning it just below the rounded curve of the top of the body. Then I used a small T-square and made sure the stake pocket was perpendicular to the top of the car, while also still centered on the pencil mark. If you do this quick enough, you have some working time with the glue. Note that first positioning the stake pocket and then coming back with the glue didn't work, because it always shifts the part. The above approach requires more speed, but it yielded better and consistent results.
Next, I cut a set of 2-foot long side flares for the stake pockets. These are made out of scale 1x2 strips.
I position these, one on each side of the stake pocket body, using a pair tweezers, making sure that their tops line up with the top of the stake body. I then apply one drop of Testors glue to each flare. This connects the flare to the stake pocket body and the car body, and also soaks the rest of the stake pocket's joint with the car body. At this point I let it dry for several hours (most of the time it was overnight), because the next step requires some rough handling.
Next, I make a pencil mark on the stake pocket at 6 scale inches from its top. From there down to the bottom of the stake pocket I file a slope. That is what close-up study of PRR FM flat car stake pockets has shown me is the overall shape of the pockets. The three outside ones are relatively easy to file down, because you have the body clearance. The inner 6 I file down as much as I can without damaging the bottom of the car body, and then use an X-acto knife to carefully hand-carve the rest of the slope. This filing and cutting leaves some flash that needs to be removed. In the photo below you can see where the first stake pocket has been filed and cleaned up, but the rest of them have only been filed.
This photo shows the first car finished. It makes quite a difference in the overall look of the car having the stake pockets installed. They took about two weeks of modeling time to do all three cars, but I think it was worth it. I can fit a scale 3x3 strip of wood inside the stake pocket, so that will work with simulating car loads rigging.