Mantua flange depth

[Blind Bruce]

I have a newer Mantua heavy Pacific with six wheel tender trucks. The tender derails every time it travels over an insulated rail joiner. Close inspection reveals that the flanges are hitting the top surface of the joiner. The track is code 83 as is the joiners.

Is there a way to correct this without changing wheelsets? The trucks are riveted together and it is difficult to remove the wheels. I need some kind of joiner at these points as they are located on a curve. 

[NeO6874]

I'm not sure if this would work, but you could try filing the plastic down a little bit, so that the flanges don't snag on it.  That'd probably be a lot easier than filing down all the flanges on the wheels.  You may be able to find replacement truck sets from NWSL (or similar company of your choice)...