I too had heard that the FM's were used infrequently on the branch. There was a short period of time that 132 was NOT the branch train, but a regular SF to SJ train. I had gotten really excited while going through the late Walter Johnson's timebooks and seeing that he had GS-4
types on 132, but at that time, this train was not the branch job. This will show up in EMP TT's in the early 50's.
As far as bigger power, the SP used F-1 2-10-2s during the War up to Permanente...these were the lighter versions and not much bigger than a heavy Mikado. Neil Vodden told me that one that he remembered was a 3600 in the teens, but could not recall the exact one. 2-8-0 2777 and 2781 were used as well as P-7 types 2476 and 2477. I remember Baldwin roadswitchers after steam was gone and the crews would go to beans at the Rancho Cookhouse late in the afternoon.