Don is right.
The gears are the same.
But the intermediate gears spin freely.
The axle gears are pressed onto the axle and this have constant pressure applied to them.
I've seen a lot of these locos. And have never seen a problem with the intermediate gears.
I would be 99.9999 % sure it is the axle gears.
I got rid of all my Arnold S-2s except one. I stripped it of all gears and unneeded stuff to make a dummy.
They are pretty free rolling,.
My plan is to make an S-2 slug one day.