Recently I added a NYC box car to a consist and noticed derailments were becoming common. It was happening over a switch that I had already smoothed out the rough high spots. After running the train over the switch I noticed that the NYC car was bouncing over the switch and not smoothly passing over it. Took the car off the rails and looked at the wheels. I noticed that two of the four axles were bent. How could that happen on a car just taken out of the box? So I pried the truck frame apart slightly to allow the axles to drop out, replaced the fast angle wheel set, pressed the truck sides back together with a C clamp, oiled and put the car back on the tracks. No more problems.
Point is don't always suspect the track and the switches are the cause of derailments. Bent axles could be the cause. As to why they were bent? Don't know but the steel that Lionel use to make these axle/wheel sets is VERY soft and they can apparently bend very easily.