My problem is just keeping those early K-Line 6 wheel passenger car trucks together. It seems that the cast studs break off rather easily, making it impossible to hold the whole truck together.
Another problem for me has been their uncoupling mechanism. Seemed like a good idea but it's far too complex and it requires a spring to return the uncoupling bar to it's locking position. that spring has broken or just lost tension on a number of my cars. I finally just got frustrated and super-glued it into place so the coupler won't open at all, just to keep the train from breaking apart while operating. Just disassembling the truck to replace that spring took an hour or mofe of fiddling around with the darn thing
Paul Fischer