I don't think this is a PBR issue as such- I think the effect is being caused by Trainz wrongly trying to smooth the transition between the end, in shadow, and the top surface in direct light. Unless told otherwise, Trainz thinks that the end and top surfaces are part of a smooth surface and it's doing its best to shade accordingly.
The edges of the faces that make up the end of the segment may need to be split from the rest of the model.
R3