The notes accompanying the link state: "With Amtrak’s decision to standardize on the Electro-Motive F40PH, the P30CH was destined to become an orphan in a world of F40PHs, a fact exemplified by Amtrak’s willingness to lease the Pooches to the Southern Pacific in 1978 for Bay Area commuter service."

My recollection is that the P30's were delivered prior to the Superliners they were intended to haul and so were made available to SP for a few months freeing up some commute-assigned locomotives for freight service. I also recall it was during the spring / summer / fall season where the Harriman's steam heat would not be missed.

Many U30's lead relatively short lives and I'd assume Amtrak also was a lot happier with their EMD's of that era.