A Line twin stack container cars?

I just bought these cars and i put them on the track with the screw scewed into both cars at the conection point. My problem is they keep derailing. I loosened the screw some and that in turn loosened the trucks some and that seemed to help. My question is, should i drill the hole bigger in the car that sits on top so the cars have a little more play.Or is this a bad idea? Has anyone had problems with these cars or am I the only one? How do you fix?[8D]

You need to post a great deal more information before any assistance can be offered:

scale, curve radius, additional wieght…for starters

My scale is HO and the radius is a full 30" and I have not added weight because right now I don’t have the containers. There is a total of five cars in the set and the two cars on the ends have couplings. The other cars are conected by a screw that comes through the trucks, up through the car with the truck and then the car that that has no truck on that side. It seems like there is no play between the two cars.