I have the MTH NS New York Coach car (20-61018) and Baggage Car 39 (from 20-65265 set), which tend to uncouple at random times in operation on one end of each car. How should I fix them?
Turn the car over, look at the coupler mechanism. If it has a spring running lengthwise, the spring tension is not enough to keep the coupler closed. You can most probably open the coupler by pulling on the end of the coupler. If so, replace that spring with a stronger spring, or elongate the installed spring, cut the excess off the spring and reinstall. You can get different springs at either Ramsey or Ace Hardware stores.