Double Stack-Tipping

[FCIN]

I'm not familiar with these N scale stack cars, but my first question would be are the wells (car body's) metal or plastic? If they are metal I would think that would be enough weight to keep the cars on the track and not give you that wobble effect. If they are plastic, then my only suggestion would be to get or modify some containers with added weight in them to rest in the well (bottom container). Also have you checked the trucks to make sure that the wheels/axles run true and aren't causing the wobble?