Maybe it wasn't a "mixed train" per the usual definition. Consider both trains involved Fort Ord during the war. In August and September, there wouldn't be any need for steam heat on the mid-California coast, especially in crowded coaches, which may not have air conditioning anyway.

The "freight" to make it a mixed train may have been regular box cars being used for baggage service, or maybe there was some military equipment as part of a deployment?

Even if they needed a steam line connection for the passenger cars - run the freight cars on the rear with a shack.

The basic design of a steam locomotive does not necessarily restrict it to either "passenger" or "freight" service. Didn't ATSF use their big Prairies in passenger and branch service? (Death Valley Scotty's run?) Some railroads (Soo Line comes to mind) commonly used Pacifics in branchline freight and mixed service. NCO and SPng used 4-6-0s in service for everything. UP used Challengers for both freight and passenger.

Did GS stand for "Golden State" or "General Service"?