IIRC before the Southern Pacific went to a block signal system, they would put red lens covers over the clear lenses of the classification lamps for when the locomotive was sitting in a siding waiting for another train to pass. The headlight would also be dimmed. It was a signal that the engine was not on the mainline and it was clear for the oncoming train to proceed.
Later engines, like the 4449, had red tinted lenses in the headlight itself hooked to a pneumatic system. The engineer could hit a switch and two lens halves would be swung together to change the headlight to red. The system was also plumbed into the automatic air brake system so that if the system was dumped or "Big Holled" the headlight would also change to red.
Cheers,
Curtis F.