125 ton trucks required on 315K cars already use 38" wheels.
I'm guessing that the larger diameter wheel reduces the peak stress on the wheel and rail. I would also guess that the peak allowable stress is set by fatigue.
Somehwere in the very early 1970's, Trains had a news item about the UP experimenting with 125 ton coal cars and noticing a very significant increase in rail wear for a given gross tonnage compared to 100 ton cars.
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