CN C40-8 Derailments Due to Consist Tilt

[wisrailfan]

I bought the CN C40-8 repaint pack from Eyein12's page and installed the locos and scenarios per the instructions. Each time I tried to load any of the scenarios (for Marias and Cajon) the rear end of the lead unit sinks into the ground and if it's coupled to other CN C40-8 units they explode upwards and trigger a spectacular derailment. If it's coupled to another unit it sinks and does nothing else. I did get an error message on one occasion that a derailment happened because of consist tilt. What does this mean and how can it be fixed? I've had no problems with any other add-on items I've installed as far as this sort of thing goes.