After the painting of the body, I could finally, permanently mount the underbody frame into the body. After two years, it seemed that they both had gained a bit of weight (probably the paint layers), so they didn't go together as smoothly, but I managed. A little bit of superglue in the corners to formalize the connection, but I don't think they'll ever come apart again. In the process, however, the end piece of the center sill, into which the coupler is to be mounted, snapped off. This turned out to be a blessing in disguise. Two years ago, I was still messing with Sergent couplers, but now I am back to the Proto Max ones. So, in reviewing the car's height, I noticed that I needed to cut the edges off of this piece that snapped off, sand it smooth, and then attach the Proto Max coupler with box and all. The part in question is shown on the lower, right-hand side in the photo below, after it was surgically altered. To be able to cut off the sides of this part on the other end, I snapped off that part as well to mount the other coupler.
I used 5-minute epoxy this time around to attach it back to the underframe. The screw hole for the coupler's box sits right below the edge of the body, so installing a screw wasn't possible. I tried to figure out how to still provide a strong connection between it and the body, when I happened upon an article in the old 3/16 "S"cale Railroading magazine where the author described how he used metal pins to reinforce connections. So, I used superglue to attach the coupler to the car, and then drilled two holes in the back corners of the coupler box, all the way through the frame. I then cut two pieces of brass wire, dipped them in superglue, and pushed them down those holes. After the glued set, I trimmed them to length.
The final detail to install are the airhoses. I trimmed the metal wire to just inside the edge of the body, and then used superglue to simply glue the detail to that wire.
The final step was to paint that detail. I painted the air line itself the same as the underbody (grimy black), painted the hose black, and the glad-hand and cock valve silver. The underside of the coupler box was also painted grimy black, while its top side and ends were painted the body color.
This photo shows that it lines up perfectly with another car.