The first diagram is just a basic sketch of the side profile of the car. The measurements are my best guess, because not all measurements are available in online documentation. Correction: the flat ends of the car, those that I have labeled as one-foot high, should be 10 inches high.
This diagram shows the bottom-up view. I made the center sill beams such that they match the width of the couplers I use, which left them 3' 8-1/2" from the side edges. Correction: the car's width was actually 9'3".
These are the humble beginnings of the three flat car models I am going to build. Note that any curvature in the photos is due to my photo-taking abilities, not the materials or my cutting abilities! I started by cutting the long sides, measuring a scale 40 feet by 2 feet. The prototype side and center sills were made out of 7/16" steel plate, which would translate into 0.006"-thick styrene in S-scale. I used 0.040", because, like the prototype, this is the fundamental frame work of the car.
It took some time, but I eventually came up with reasonable measurements for the side profile. I decided to make the skinnier outside parts 1 foot tall. The prototype's dimensions are 10", but one foot makes for easier work.
I then used the score and snap technique to break away the sections not needed. This was not trivial, and left some rough edges, as you can see in this photo.
I came up with the idea of clamping all six sides together and then filing them all even. This is where making more cars at the same time comes in handy.
With the long sides done, I cut some scale 4"x12" strip styrene to the length needed to make the assembly 9 feet wide (correction: note that the prototype cars were 9'3" wide). The end pieces are glued in between the two long side pieces. This photo shows my set-up for making sure the end piece is perpendicular to the long side.
After gluing the other end piece, I had the basic frame. The center sill pieces, in the prototype, were the same "fishbelly" shape as the outside side sills. It took so long to carefully mark and cut the side pieces (which could not have a cut line showing), that for the, mostly invisible, center sill, I decided to make it out of two parts, starting with a 12"x4" styrene strip. I glued them centered, and at such a spacing to match the width of the coupler boxes I am going to use. Next, I took another piece of 12"x4" strip styrene, marked the sloping profile and then cut that off (I was able to use the NWSL Chopper II for that angled cut, which goes a lot faster and is more accurate). It was then just a simple matter of gluing that separate piece on top of the center sill strips to create the same profile as the outside pieces.
On the inside of the ends, I glued a piece of 8" channel strip styrene to reinforce the ends. This photo shows the front car upside down, and the rear car right side up. The channel is flush with the bottom of the car.
Next, I cut and glued some strips on top of those 8" channel strips. The prototype seems to have three beams going across length-wise. If I had had enough material I would have used 4"x6" strips every where, but I didn't. So, I also used two strips of 4"x4" and 4"x8". These are going to be hidden from view when on the layout, so it isn't that big of deal, but it is something to remember if I am going to make more of these cars.
Although the car is fairly square, some additional reinforcements are necessary for the model. The prototype had two crossbearers at 5' from the center of the car. I did not have that prototype information at the time I built this, so mine aren't in the correct spot. I first glued some 4"x8" strips under the length-running strips installed above. These approximately match the prototype (update: I have since that time found additional photos, and I should have placed these beams such that they correctly support the brake system parts later on). With those dry, I glued a 12" wide channel strip in between the center sill beams, to complete the overall shape of the center sill. I am using the weights to hold all of it together while the glue sets. I now have a truly square car frame.
The prototype shows some heavier beams going across right where the angled section of the side profile starts. I made these out of several strips of 8" wide styrene (two 8"x8" and one 4"x8" to total the 20 inches of interior depth of the car). This really ties the center sill to the outside side pieces and solidifies the car. I then cut some 2-foot long strips of styrene and filled in the area in the center sill where the truck mounting screw will go later. The center of that needs to be 5 scale feet from the ends of the car. Needless to say, the issue with a basic flat car is that it is going to be extremely light, especially one made out of styrene. I therefore used several tubes of superglue and glued a bunch of lead shot in the center sill, making sure they are not visible when the car is viewed from the side. The car definitely feels heavier with them in it.
So here we are. Three basic body shapes. There is more under-the-body infrastructure to add.
More cross bearers have been added, somewhat copying the prototype.
I also added diagonal beams at the ends/corners. I think this is about as far as I am going to take the underbody work; most of it will not be visible anyway.