Here is something we generally only see on those old Hornby clockwork loco wheels 'Zink Pest' This is the first time I've seen it on a more modern die cast model, a Bachmann Hong Kong made American thingy....
The chassis expanded to the point that the plastic body had no choice but to split and give more room for the expanding chassis.
https://en.wikipedia.org/wiki/Zinc_pest
Eoin