Run the carriages very slowly into the curve where they derail and watch them very carefully. I had a similar problem with some carriages and discovered that the trucks were causing the wheels to come off the rails. The trucks in question seemed to get stuck and have something which interfered with their movement. I worked the trucks and lubricated them with silicone which seemed to solve the problem. Others have noticed that a carriage at the end of a consist with a slider at the very end would also have a derail problem. The slider seems to force the back of the carriage off the rails. Moving the slider to the front of the carriage seemed to resolve the problem. If that is not possible I would add weight to the carriage above the slider to see if that helps retain the truck on the rails. The other problem encountered was that the slider would short out the layout. If moving the slider is not possible then perhaps replacing it with a shorter one might work.

Yes Jean, sometimes the shorter shoe is better. I use inexpensive lead soapbox derby car weights that come with self stick strips. They separate at the seams so you can double the thickness if needed. Acrylic paints work fine to match the interior colors. They usually fit up against the corridor ends behind the windows or inside the water closests. On some cars, they can be put on the floors.