On the SP, those were "Train Indicator Boards" where the train number would be displayed, rather than the locomotive number. The UP followed the same practice.
The story, as I understand it, was when the number boards were located near the front of the locomotive, the train numbers were virtually ureadable at night due to glare from the headlight and Mars light, so they were moved amidships.
When the 4449 was first restored, the number boards were left amidships. Doyle & Co. eventually moved them back to the front, probably more for esthetic purposes more than anything else.
Rusty
