to Collyn:
How sharp are your curves? If they are 18"-radius and you are trying to run BIG locomotives like long 6-axle diesels, more often than not you will experience derailment problems, particularly at the rail joints where sometimes there is a kink, or if the rail joint has a sharp edge on the inside where it will catch a flange and throw it off the rail.
Also, if your curve leans outward, derailments are even more likely. Curves should lean slightly inwards.
Due to space constraints, I am forced to use 18" radius curves, and I also get a lot of derailments with long 6-axle locos like SD40-2s. I had to:
1 - Lay the curves using flex track to minimize joints.
2 - Make sure the joints that I cannot eliminate have no sharp edge or kinks.
3 - Superelevate the outside of the curve by shimming it with pieces of .015" styrene.
4 - Ensure my locos have free-swiveling trucks and that they are not warped, the wheels are in gauge and in good shape.
All these things together made my derailment problems virtually disappear.
Hope this helps!