A few thoughts:
1) The spring under the pilot truck may be too stiff, thereby reducing contact force on the front driver enough to cause it to climb over the turnout rail. Easiest way to check this might be to remove the pilot truck and test. If it improves, reduce spring tension on the pilot truck by slightly compressing or shortening the spring.
2) Add weight inside front of locomotive and test.
3) Interchange the switch with another on your layout and see what happens on both switches. I know that's a lot of work but...
4) Incorrect driving wheel or pilot wheel lateral spacing?
MELGAR
