Any gearbox experts here? Just puzzling over why really big gearboxes are commonly avoided. Locomotives mostly use electric transmission and the reliability of gearboxes in things like wind turbines isn't great.

Is there some reason to do with scaling the geometry, like if you double the size of every dimension, the shaft can transmit more torque than the teeth? At a quick glance, it doesn't seem that simple. Or is there more slip between the teeth and more wear? I can't figure out a concrete reason and it's bugging me :-).

Chris