Printing a Freight Car: Southern Pacific CS-35A Flat Car

I’ll admit it. I’m not a freight car modeler. Sure, I’ve had an appreciation for appropriate freight cars from my era, and I’ve built a lot of resin kits for the Vasona Branch. But I’m not one of those folks who could argue the difference between the different patent pressed-car ends, or could tell you what percentage of B-50-14 boxcars had Bosendorfer side doors, or would argue about the appropriate underframe for a stock car. But, hey, all that goes out the door when I decide to print freight cars on the 3d printer.

My list of potential 3d-printer projects has always included a few freight cars. As popular as the Southern Pacific may be, modeling the 1930’s means building a lot of resin kits, searching eBay for old kits, or finding a lot of stand-ins. One example would be the Hart convertible gondolas, frequently used for dumping ballast on railroad tracks. Another would be flat cars - Southern Pacific’s 1930-era flat cars look much more fragile than the typical 1950’s plastic flat car kit that’s available, and tended to have a very specific look. More importantly, I needed flat cars for the Vasona Branch - lumber coming from the sawmill at Laurel needed to come down the hill to the lumberyard at Auzerais St.

I’d hoped my first-generation Makerbot was going to help me with freight cars, but it couldn’t print something that looked good at HO scale - the layers in the plastic were too obvious. The Makerbot could only print within a 4 inch cube - not big enough for even a smaller flatcar. The Form One, though, was getting big enough. At 125 mm x 125 mm x 165 mm, the printer could print a 36 foot freight car diagonally, or a longer car if I tilted it up on end. So, soon after I’d gotten the Form One up and printing, I tried a couple flat cars. Here’s the details on that first flat car.

The Prototype: Southern Pacific CS-35A Flat Car

In 1903, the Southern Pacific decided to experiment with freight cars built completely of steel. Two plans appeared that year: the CS-35 car (36 feet long with 40 ton capacity) and the CS-35A car (40 feet long, 40 tons). Both these CS (Common Standard) cars represented first attempts at a flat car with a steel under frame: a pair of 15 inch I-beams along the center, and pressed steel sides tied together by a 19 inch deep C-channel. The two designs were apparently in a bake-off; both designs date from October 1903, but only the CS-35A got the green light. SP had 1,000 of the 40 foot cars built by the Pressed Steel Car Co. in Chicago.

The CS-35A, designed during the Harriman era, differed from later flatcars such as the F-50-2 by having only nine stake pockets on each side compared to eleven or twelve on later cars. Other spotting features was the lack of rivets on the bottom edge (unnecessary in the stamped forms of the early cars, but needed in the fabricated side frames of later cars) and twelve rivets on the side at each truck bolster.

These flat cars definitely looked different from the other standard SP freight cars - relatively shallow sides, no truss rods, and longer than most of SP’s flat cars. Compared with modern freight cars, they look positively wispy and fragile. They also lasted a long time, with Tony Thompson’s SP Freight Cars book showing the cars still in common use in the 1920’s, both for revenue service and for maintenance.

Of course, there’s no models of the CS-35A available, whether as a resin kit or plastic model, so it was a great first try for 3d printing.

The Model

The first question was how to build the model - do I try to print the whole freight car in a single piece? I’d seen models on Shapeways where only the metal parts were 3d printed; the wooden deck for the flat car was done with laser cut wood. That seemed great; the wood deck would be more realistic, and I could print the model upside down on the build platform. (That didn’t work; more later.) I tried doing a couple models where only the frame of the flat car was printed, with big openings below. That didn’t work - the printed piece ended up being too thin to print, and wasn’t strong enough to survive removal. I ended up building the model so the top was solid and 6mm thick; the wood deck would be glued onto that surface.

I did the initial drawing for the CS-35 in SketchUp. I started out sketching the rough shape, extruded it as a 6 scale inch thick slab, then started adding details - first the frame and ends, then more and more detail. I scaled the sides so they were 6 inches thick and printed more reliably. For details - stake pockets and patterns of rivets - I often drew the objects in a separate SketchUp model, then made the object into a group and pasted copies into the final model. I learned that 1” diameter cylinders, 1 inch high, made great rivets. I also marked the holes for grab irons by drawing a 1.5 inch diameter hole.

There were also details I didn’t model. I didn’t add brake gear to my initial model. I planned on using wire grab irons, and grabbed brake wheels from my scrap box. I did need to model the brake cylinder for the car; it’s obvious on all the real photos I’ve seen. I started using the Tichy Train Group’s K Brake kit, and bought a half dozen at my local model railroad shop, but found I was only using the brake cylinder. I ended up building a 3d model for the brake cylinder, and printing the brake cylinder myself.

One of my other surprises when building this was the question of what exactly to build. Any model has compromises - I might not want to add all details, or I might need to change a dimension, or I might tweak the model to fit a model railroad coupler. Figuring out these differences between the model and the real thing is the hard part of making a model, whether by hand, with a 3d printer, or when designing an injection-molded kit. For the CS-35A, I had to decide whether to add some of the underbody beams and braces (no), or what detail to add to the ends (minimal.) I also had to choose between scale dimensions and a reliably-printing model. I ended up making the center I-beam much thicker than in real life so it would print more reliably. Instead of 1 inch, I made all the walls 2 inches thick. I also drew it with slopes inside the I so there wasn’t a difficult transition from thin to full-width.

Flat car and support structure. Support structure holds car at 30 degree angle to avoid any horizontal slice through the middle being too large.

I also had hiccups. I had problems printing the final model because of extra hidden faces; I used Netfabb Basic to clean up the models and prepare them for printing. Even that didn’t work reliably. Because of the Form One’s resolution, making a “watertight” model (no holes or gaps between faces) was critical. On the Makerbot, the low resolution meant holes would usually be ignored when breaking the model into layers. With the tinier resolution, even a small hole could convince the Form One software that you intended the model to actually be hollow. I also found that pieces thinner than 2 scale inches (0.020 inches, or 0.5mm) didn’t print reliably. (The design rules from Shapeways for their frosted ultra detail material have similar limitations.) I found I couldn’t print the freight cars flat on the build surface either without layers failing to bond. Formlabs, the maker of the printer, suggests that large flat prints be done at an angle so that any individual printing layer doesn’t cover a large area. Doing so gave me more reliable printing, but doubled print time. I also found I needed to be careful about cleaning the model - I’d been leaving the finished prints in direct sun to cure, but had problems with warping. Keeping time in the sun short, and making sure both sides were exposed to the sun helped limit warping.

Initial model. Note messed up corner caused by deformation when printing near edge. The other cars weren't this bad, but all cars printed with the long edge tipped had a bit of deformation.

The overall process for printing the flat cars was something like this. First, I’d print a couple models over a day or two. Like all the 3d models, I’d pull them out of the printers, remove them from the build platform, and soak them in isopropyl alcohol to wash away the extra resin. I’d remove the support structure added to print the model, then leave the models in the sun for 10-20 minutes on a side. Another day, I’d spray paint the models (Scalecoat boxcar red in a spray can). I’d drill out holes for the screws holding on the trucks and couplers, then attach both with plastic screws, and cut the screws off flush with the top surface. I’d then add a wood deck using individual 2x8 wood boards stained with india ink and alcohol, gluing them on with contact cement. I decaled the cars, then then drilled holes for wires representing grab irons and superglue them in place and touched up the paint on the grab irons. A final spray of Dullcote sealed the decals onto the model.

I made a couple embarrassing mistakes with the model. On my first try, I used the plans in Tony Thompson’s “Southern Pacific Freight Cars” book to build the CS-35 36 foot car. That model turned out well (and the 3d model is available on Thingiverse). When I went to decal the model, I looked for the car numbers for the 36 foot cars… and realized there were none listed - SP never built any. I'd been printing the Loch Ness Monster of SP flatcars.

To correct my mistake, I stretched the design out to make a 40 foot car. In SketchUp, this wasn't too challenging; I'd select the portions of the flat car from the bolster out, then use the move tool to drag these out the appropriate distance. After I printed a few of the longer cars, I compared them against photos, and realized the model didn’t look right - the wheels were set closer to the center of the cars. The CS-35A looked a bit like the ends of the cars were unsupported. I went back to SketchUp, and moved the truck bolsters in.

The final model requires about 15 cubic centimeters of resin and prints in three and a half hours. The resin cost about 15 cents a cubic centimeter, and about half gets lost during printing and cleaning, so we’re talking about $3 in resin per freight car. Compared to Shapeways, which would charge around $30 for a similar part, it’s not a bad deal. On the other hand, I went through a lot of prints for testing - probably 10 to 12 pieces. Once I got the design and printing settings done, it was easy to print more flat car castings on demand.

I still have some minor problems with warping, and the arrangement of the model usually causes one corner to be bent in a bit. Both they’re still fine models,and I’ll be able to make some reasonable 1930’s flat cars for my layout quickly. One potentially big problem is that the models are very light - only about 1.5 ounces with trucks and couplers, much lighter than the recommended 3.5 ounces. I’m planning to either add weight to the loads on the flatcars, or add some lead sheeting in parts of the model.

The Decals I printed and decaled one of the “wrong truck spacing” cars using a mix of leftover details; that process was amazingly painful. I tried finding some commercial decals that had the needed freight car numbers and tiny lettering, but didn’t find any good sheets at my hobby shop. I ended up deciding that custom decals might be reasonable, especially if I was going to build ten cars.

I drew up the artwork in Intaglio (a vector drawing program like Adobe Illustrator). I tried using Ben Coifman’s Railroad Roman font which I’d bought years ago, but its lettering looked a bit thinner than the stock SP lettering style. I ended up doing half the sheet using the Baskerville font available on the Mac which resembled the SP lettering, but had thicker letters. The bolder glyphs should make it easier to read the cars in my dark garage.

I sent the decals off to Rail Graphics; my 1 inch by 4 inch decal sheet cost $50 for a run of 50 decals, each with enough lettering to cover a few cars. The finest 2 inch tall lettering isn’t legible, but all the other lettering was readable and fine for my cars.

I’d never seen any hints on making custom decals, but designing my first sheet gave me a few lessons. First and most important, I made sure that the car numbers included a bunch of optional numbers that matched the real cars. For my CS-35A cars, car numbers would have been between 78500 and 79499. I added several numbers starting with 78/79, and also made sure that numbers starting with 48, 49, and 40 existed in case I wanted to do some of the other SP flat cars. I added several different car lengths and weight capacities so I could do some of the alternates. I also added the SPMW lettering in case I wanted to model any of the maintenance of way cars seen in photos.

I didn’t think about the difficulty of cutting the decal apart. The lettering on these decals was remarkably tiny; I had to borrow my wife’s Optivisor to be able to cut them apart. I put the “feature” lettering - “dirt collector”, “metal brake beam”, etc. on a single line, so it was easy to cut a strip out, then break out the individual words. For the other lettering such as the reweigh information, I did a lot of cutting out individual words. I wish I’d put most of the data for a single kind of car on a single line so I could cut that line off the decal sheet, then cut the individual pieces off separately.

Oh, and if you decal one car by hand before doing the custom decals, don’t use that number on the decal sheet.

The Final Models

So far, I’ve built six of these cars - two of the non-existent 36’ CS-35 cars, one of the CS-35A cars with the wrong truck spacing, and three of the 40’ CS-35A cars with the correct truck spacing. I also have one model that I had printed by Shapeways in Frosted Detail (the lower-quality.) They’re fine cars for my layout. They look positively fragile next to the chunkier Athearn 40 foot flat cars, and I also know that they’re historically accurate.

What’s next? I also like the look of the SP F-50-4 cars, with their narrow sides but prominent center beam. I’ll show more of my progress on those later. I’m also seeing about building models of the W-50-3 Hart convertible gondolas used in ballast service. Keep your fingers crossed that I can print those reliably.

And finally, let's check out the cars in action: