Design is an iterative process. I could have use the printed shells as is, but I prefer to refine the design a little bit so it can fit my needs and others much better.
The car ends were reworked a little bit to look better and be easier to produce. A few prototype pictures also shown I didn't use the right brakewheel housing, so this issue was addressed. I tried molding the brake rod directly on the model like an old blue box era car, but it didn't look great. It will be easier and more realistic to simply insert a small L-shaped wire.
The space between structural members was also a little bit too tight for a Kadee #242 coupler pocket and I adjusted it to fit perfectly. 3D print is great when you can keep preparation work at a minimum. I also alter the coupler height to conform to NMRA standards using a Kadee coupler gauge.
The next big change came later when I tried to fit weight on the underframe. My initial thought was to fill the space between the members with steel shots. However, after testing how much weight could be added, it was clear it was a far cry from basic NMRA recommended practices. The cars should at least weight 5.5 oz each once completed. Right now, the shell with trucks, couples and metal wheels is about 2.5 oz. I need 3 oz more and it was clear that could only be achieve with lead.
I have a box full of Select Xtras lead wheel weights part #FSL03 laying around. A quick test shown me they would fit perfectly between the main underframe members and be completely invisible... excepts for a caveat: the intermediate members would need to be removed. Given underframe details are less important in my eyes than a great looking gondola interior, it didn't take me long to remove the extra members from the 3D mode. It is certainly less accurate, but at least it doesn't impact the brake rigging, which is much more important because it can be seen under certain circumstances. On the positive note, an entire wheel weight stripe can be used per car, adding exactly 3 oz as required.