Finicky Trucks on a Locomotive?

I have one of these and it has always tracked just fine - which makes me ask: is there something on the track itself that the loco/trucks are sometimes encountering? You stated that the area was not easily observable.

Maybe a dropped screw or detail or coupler thumbtack? Has a piece of track wandered but is not visible? Have you ever actually run this loco much?

Always annoying, these "but I changed nothing" issues. 

Loose side rod linkage?  If one of the hex screws is loose, the eccentric may be hitting a tie and pole-vaulting the loco off the track?  Just thinking aloud here.  Depending on how careful you were about making your track joints smooth, you might check to make sure the driving wheels are in gauge.  If they are gauged a litte wide, they might be snagging in a track joint.  That's about all I can think of.

Looking at the parts breakdown for this engine it shows conical springs on the pilot and trailing trucks. Maybe the springs have relaxed a little over time and not putting enough down pressure to keep the trucks on the rail. I would try stretching the springs a bit.

Doug

Thanks guys. I will try these tonight!

I have a Santa Fe Hudson that occasionally derails on curves. It seems as though the trailing truck gets stuck in a certain position and will not slide sideways easily enough. The previous owner greased the slides, but that did not do the trick.