I did a lot of experiments with magnetic tension locks and while they were cheap, fun, and pretty effective, I ended up getting annoyed at how big and bulky they were. I ordered some 3mm scale Sprat and Winkle etches from MSE to give them a try, and I liked them even more - especially since one 3mm diameter Neodymium magnet is enough to operate a pair of couplings, albeit without much margin for positional error.
Teeny weeny! I drilled suitable holes and the magnets fit in. They're recessed just enough that a few grains of ballast will fit over the top. Thankfully this means no removal of sleepers and making good after, apart from one siding where I experimented with several different magnets and have a big slot to fill in.
The intent is to fit Sprat & Winkles to the ends of wagon rakes, and use 3 links (Smith's Instanters at the moment) between wagons in the rake. A few wagons (e.g. a couple of vans, and an OCAs or two) will get S&Ws both ends so they can be shunted into other trains. I've seen a couple of pictures of Railfreight red OCAs on the back of autoballaster sets, for example, which should make for a nice contrast.
It feels great to have finally got my act together and made a decision about the couplings - hopefully I can maintain the momentum!