Mike, Assuming you have reasonably decent track and have cleared any debris from the middle of the tracks, the easiest fix is paint them!! Again I stand with JD on this issue, I spray paint my Bachmann Knuckle Couplers with the cheapest Wal-Mart or any discount store red oxide primer. The added bite from the paint seems to do the trick. They not only stay together better and save me all the work of the other fixes, but also look great in the rust like color of the red oxide paint. I keep several complete trucks with the couplers painted and installed in stock. When I get a new car I swap out the trucks and put the new ones in the paint shop til I get ari=ound to painting them. Sometimes I also remove the wheels and paint the truck sideframes as well. I get lots of complements on the cars I have done this to. If your mine cars do not have metal wheelsets, add them to add weight to the cars. I have watched a 45 tonner pull 16 of these cars on good track with no problems!!
Hope this helps a little.........
Hope this helps a little.........