All my model railway layouts use Spratt & Winkle couplings. These rely on a steel chain that the uncoupling magnet attracts. In 4mm scale I can use the stuff supplied by the EM Gauge society.
3mm is different though. Here I have to make my own by winding wire around a bit of rail and then slicing through the coils with a piercing saw. Then each link is joined up and hung from the etched coupling.
Occasionally the magnets do more than uncouple, they grab the last link on the chain. Once that has happened the things don't work any more - 3 links are required to be within grabbing range. Therefore I take the pot containing spare links to shows and replace as required with some long nosed pliers and a bit of fiddling in the fiddle yard.
At the weekend I realised it's time to make some more chain up - my stocks are nearly exhausted. I'm back to searching the magnets for missing links, not all that successfully either, heaven knows where they go. Maybe the magnets are eating them ?