I walked the line several times on the last weekend of operation. While not an expert, the ties looked good to me. In fact, in that exact area, I measured the tie spacing to ensure that I got it right on my live steam layout. That section is well ballasted, properly drained and the ties looked good, but it is a very sharp curve on a steep grade.
Some combination of high axle loading, long rigid wheelbase, coupler leverage, or pre-season gauge widening are possible candidates. Another is a "kick in the pants" by the steamer if the diesel lost traction for some reason.
Trying to make it look like it was GLRR's fault by blaming the ties is sort of like a politician blaming his predecessor for every thing that goes wrong during his term!