I own 7 of Lionel's Standard O Husky Stack cars and I have only 1 TTX car where the coupler keeps coming apart during operation. As mentioned, the problem seems to be confined to just the one car, and I have already tried replacing both trucks. One seems to be working great, but the other is having the same issue. I'm running with a few 0-45 curves so unfortunately need to use couplers long enough for these cars.
Any advice/solutions much appreciated! Enjoy your Saturday!!!