I believe what you speak of are "Train Indicator Boards", Espee was a frequent user of them. The purpose of them was to display the train number, ala' "115" or "116" for the San Juan, or in the case of the Silverton "461" or "462".
Since most of the D&RGW ran as unscheduled extra trains, the white flags would be flying, and the number indicators would read "X-488" (an example) for the Extra 488, based upon the number of the loco that was assigned as the road engine.
When 844 and 4449 doubleheaded in the Pacific Northwest a couple of summers ago, the crews messed with these boards on the locomotives, whereas the 844's said "X-4449" and Verse Vica.
William Diehl... \M/( >.< )\M/
-Albuquerque
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