To clarify: on railroads which displayed schedule numbers on the locomotives, like the SP, what you call "number boards" are technically "indicators". For regular trains, these are used to display the schedule number the train is running as, such as "98" or "223". If the engine had been authorized as an extra train, it would display it's number preceded by an X, such as X3795. By the time this picture was taken, the SP had stopped using indicators on all trains except the Peninsula commutes, which is why there is no X on the 3795 and the numbers are permanently painted on the plastic insert. The small doors weren't usually removed.