Bill,
I only have one switch which has a straight track leading into the switch. That switch has not yet presented any problems. The problem does happen with one switch where the curved track leading in and the curved portion of the switch go the same direction (so not an S-curve). It also happens with a couple of switches where the configuration is that the curved leg of the switch goes in the opposite direction of the curve leading in (so the loco would be taking an S-curve if it headed out of the switch on the curved leg). The loco actually does fine negotiating these S-curves. With these switches the derailments occur when the loco is continuing through the straight leg of the switch. Basically, when the loco is coming off a curve and enters the switch the flange of a wheel on the trailing truck (never the first truck through) rides up on the point rail that is against the outer rail. The weight of the loco is pressing up against this wheel and the flange catches the point rail just right to ride up on it.
The trucks do seem to pivot fairly freely.
Thanks again!
Derek
