I thought I'd fit some S&W couplings to the layout wagons - they are what I use on layout after all!
First, you need a setting up track.
I start by glueing a de-headed dressmakers pin to the buffers. Yes, I know it should be a length of wire and that should emerge from the buffer beam in a U-shape, but this is "quick'n'dirty" coupling fitting. Not finescale, but it works, especially on tight curves where the wire on buffers method allows the coupling hook maximum side slideage.
Superglue is the adhesive for this job. A blob on each bufferhead and then roll the wagon up to the jig on which the pin is balanced.
Next, the hook. This is released from the fret, bent to shape and chemically blackened. You can use paint, but it will chip off. Guess how I know this...
The instructions suggest some bent wire but that never worked for me. Instead, a re-bent staple is pushed through the plastic floor with a soldering iron. Quick, simple and reliable. Practise on a bit of plastic sheet until you get the hang of this.
Once in, the tails inside the wagon with some cutters and you can be confident the thing won't fall out. With practice, you can even re-heat the staple to adjust it.
Finally, some testing. The chain is attracted by the magnet, pulling the coupling hook down. The chai is from the EM Gauge Society. Not sure if it's still available as I bought several feet years ago and haven't used it all up yet.
Fitting these takes minutes but I find them 90% reliable in use and very robust.
Sprat & Winkle couplings come from Wizard Models.