I would be very suspicious of the longitudinal movement of the trucks.  This is an adjustment that is very often neglected.  The truck may swivel very easily but it is essential to check the longitudinal movement.  I have adjusted all the trucks on my Shay.  This is done by removing the truck and using a drill bit that fits snugly into the recess where the screw head fits.  DO NOT USE A DRILLING MACHINE.  If you use a drilling machine the plastic will pickup and you will drill right through the truck.  You rotate the drill by hand and remove a SMALL amount of plastic.  Refit the truck and check the longitudinal movement.  The trucks must be able to rock backwards and forwards.  If there is no movement in the longitudinal direction you will get derailments.

This adjustment is important for all rolling stock and locomotives.