MTH 4-6-2 Derailing on switches

It is derailing while traveling on the diverging route? Does the rear truck side easily side to side? Maybe  a little grease? If it has never done it before I'd look at it sticking. If it does it going straight thru I'm lost!

Does this happen on all of your switches?  If it does, then there is a problem with the engine.  

If it happens only with one switch,  I usually find that something has changed on the switch.  The problem can be a small piece of debris on the track, between the guard rails, etc.  Sometimes I have to look at a switch where it is derailing with a magnifying glass to see the problem. Another trick is to use compressed air to blow all around the switch.  This sometimes corrects a debris problem that you can't find.  You didn't say if the track is ballasted or not.  Loose ballast can cause this problem and can't be readily seen.  

The switch or the engine's wheels may have become out of gauge.  Usually the track becomes out of gauge before the engine's wheels.  Also, make sure the engine's traction tires, if any, are OK.  I have had loose or damaged traction tires hang up on switches for no apparent reason.  Make sure that both drivers have their traction tires.  The tires should be all on or all off.  I take traction tires off my steamers.  

I have had a piece of track move out of gauge when an engine is on it and pop back into gauge when the engine was off the switch.  This is really hard to find.  Try to manually move the rails side to side and up and down.  Make sure that they are correctly spiked down.  

Run the engine slowly through the switch and note the spot where it jumps.  You will most likely find a problem near that spot.  

Good luck.  Please let us know what you find.   NH Joe

Thanks for the replies, so the engines rear trailing truck ( the 2 wheeled axle) is derailing. It happens more often trying to go straight through the switch on the oval clockwise as that is the direction of my mainline, but does occur on the diverging route to the yard as well but that's harder to catch since its much slower speed vs the mainline which I like to run 40 SMPH. At 5 SMPH the trailing truck doesn't derail, anything above it does. Now with that being said when the engine is running in reverse going clockwise there is no derailment. I ran the steamer counter clockwise (in the forward direction and reverse direction) and she runs through both switches no problem even at my prefered 40 SMPH and higher just like she used to before I had this issue last night.