There's a book in SP dieselization, but that's one I don't have. I don't think it was as much WW 2 as it was where the railroad was spending its money. The E-6 was only produced from 1939-1942. Ending production was probably a war production board decision. It wasn't suitable for freight and the government wanted dual service locomotives. It also allocated where diesels went. In the pre-war years the SP was spending a ton of money on its new Daylight and Lark train sets, and the decision had been made to use steam to pull them. Iron Horseman is correct, they did jointly own the "Cities" fleet, but that's all. Their six axle passenger fleet was mostly E-7's; E-9's and PA's bought after the war.