UP also extensively rebuilt one of the ex-SP plows about 10 years ago and had it fail the first time it was out. The issue was bearing failure. SP used plain bearings and on the rebuild UP put in roller bearings. Unfortunately the mechanical engineer that picked the new bearings did not know how much of a load the Sierra Cement puts on the blade. The overloaded bearing self destructed. UP had to continue plowing with a non-rebuilt plow.
In the video "The Battle For Donner" filmed in the early '90's there is a scene in it where you see the SP guys packing snow around the bearing housings to cool them. It worked and they kept on plowing.