Basically there are 3 areas that are the problem:
You haven't really given very much information on what the circumstances are so there isn't much concrete that people can do. Is it actually derailing on a switch? Is the engine derailing on a facing or trailing point move? What size switch is it? #8 or #4? Is the engine entering the switch on a curve? What radius? What is the minimum radius of the curves on the layout? What type of track are you using?
Before you start cutting up the engine or modifying it, you need to figure out where its derailing, if its a track problem you could completely mangle the engine and still not fix the real problem.
1. The track is bad. If the rear truck derails at every switch or "random" switches it is probably not the track, UNLESS all the switches at which the engine derails are the same type of switch, for example they are all left hand #4 PECO switches. Check the gauge, that the points are fitting up, that the switches are flat, etc. If the train is derailing before it gets to the switch then its obviously not the switch, but it still could be the track. In that case look for narrow or wide gauge, kinks, mismatched joints, too tight of radius (anything below 24" radius should be gone over with a fine tooth comb), dips in the track or places where there is bad crosslevel (one rail lower than the other).
2. The wheels on the rear truck are bad. The rear truck wheels could be out of gauge, mounted crooked or binding.
3. The rear truck is not functioning properly. The truck may have its swing limited by the details on the side of the engine or under the cab could be limiting the swing. There could be a spring missing or out of place.