This is an HO scale flat car model of the Southern Pacific Railroad's first steel flatcar, designed around 1903. The CS-35 was a 36 foot long flatcar, built out of fabricated steel shapes. The SP designed both this car and the CS-35A 40' car at the same time in a flat car bake-off. The 40 foot car won. None of this CS-35A cars were ever built.
On a good resin printer, this model is approaching the quality of injection molded plastic.
I printed these flat cars on a Form One resin printer, printing the cars upside down. After each car was printed, I fixed bubbles in the surface, tapped the coupler and truck holes for 2-56 screws, and painted the models. I made wood decks from individual 2x6 HO scale lumber stained with india ink and glued on with contact cement.
For additional detail, I added a brake wheel on a piece of wire on one end, and got an HO casting for a KC-style brake cylinder from the Tichy Train Group #3005 brake parts kit. I used Kadee trucks and couplers held on with plastic 2-56 screws.
The 36' car will fit diagonally on the build surface. A 40' car is too big, though; it needs to be tilted at a 30 degree angle to fit. Although the car fit, printing the car flat on the build surface didn't work well. I ended up with large voids in the sides; I suspect that the extra "cooking" done to stick the first layers to the build surface somehow kept higher layers from curing. I now print all these cars using the standard Form Labs support structure.
Plans for the flat car come from Tony Thompson's "Southern Pacific Freight Cars Volume 3", published by Signature Press. There are a couple photos of the CS-35A flat cars, and plans for the CS-35 36' and CS-35A 40' car in the book, though you'll need an imagination and a magnifying glass to read some of the numbers.
More details and video here: