It is a bit of a convoluted story, but Bob as it right, i.e. it was all about costs. The Union Pacific was forced to take some, if not all of the C&O 2-8-8-2 locomotives, by the War Production Board, in order to help handle the increased WWII traffic volumes over the western mountain grades. The UP didn't like the C&O H-7 2-8-8-2s, as they were too slow for their needs, and as soon as WWII was over, and freight traffic levels subsided, the UP immediately took all the former C&O 2-8-8-2 out of service, and naturally the C&O didn't want them back.
The Souther Pacific made a deal with the UP to purchase those big C&O tenders, for scrap price since the UP was going to scrap all those 2-8-8-2s anyway. The SP then ran all those former C&O tenders through the Sacramento Shops and converted them all to oil, many of which were then placed behind some of the AC-9s as they were being converted to oil, while some more of those former C&O tenders went behind SP F Class 2-10-2 locomotives in order to increase THEIR water & fuel oil capacity.
The end result was, some of the SP AC-9 locomotives retained their original coal tenders converted to oil, while other AC-9 locomotives received the former C&O tenders. The easiest way to tell, are the tender trucks; original SP tenders have Buckeye 6-wheel trucks, while those former C&O tenders have Commonwealth 6-wheel trucks.
