S-scale Gunderson 48' container cars used to be available from Des Plaines Hobbies, but they currently are not. Randy Griggs wanted to add some 53-foot cars to his fleet (which he might have kitbashed from the 48-foot ones), so he found a 3D CAD design .stl file on the Internet of a reasonable approximation, modified it to S-scale, and printed it out. He found a brake wheel from another 3D source as well as an A/B valve to add to his model. Randy says it takes about 20 hours of 3D-printing on an Ender 3-Pro filament printer to print the entire body in two sections. Randy also made his own decals following the TTX prototype (the ones shown in the photo didn't come out black enough, which he has fixed since).
The next photo shows a close-up of the brake end of the car.