This seems to be a problem with my steam tenders. They pick a point and come off the rail and by the time the engineer has it re-railed the tender has uncoupled from the draw bar. Unfortunately the engineer doesn't always notice and now we are pulling the tender and cars with the decoder wires. Even I have been guilty of this. Some of mine come off easier than others. I have wanted a fix for this for some time and could not come up with an idea that would make it easy to uncouple the cars when you needed to. I do not uncouple them often but I did not want to make it a chore. I tried a couple of different sizes of styrene tube but by the time I got it to fit it would not stay on very long. I was looking at a piece of wire on my bench the other day and thought maybe it would work if I stripped the insulation off. The loco I was working on required an ID of .09" to cover the pin on the tender. I came up with a 12 guage wire and stripped off a piece, expanded it with an awl and I could slide it on as a tight fit. It is cut to be about 3mm long and I coloured it black with a sharpie marker. If I need to take it off I can just pop it off with a flat blade screwdriver. The next tender needed something smaller so I went down to a 16 guage wire. Some things are easy once you get there.
This is before painting. There is still room for the draw bar to move up and down.