The way I recall it, the locomotives delivered from about 1966 or '68 until the removal of strobe lights had a circuit breaker labeled SIGNAL LIGHT that controlled the strobe. Most of the older retrofitted units, though, were wired so that if the m-u control switch was placed in other than trailing position, the strobe light was active when the engine run switch was closed. The bicentennial units had two strobes, one on the left and the other on the right. There was a rotary switch in a box added to the control stand to select how they were to flash. Once Santa Fe decided to discontinue using strobe lights, they all disappeared within about 30 days. I have never seen anything done so quickly by the Mechanical Department. There must be more to the story.
Nobody missed them. They weren't very effective as warning devices.
Anyway, Mike, they were normally working on the lead unit when it was on a train, moving or stopped. Some of them would be on when the engine was in the roundhouse or diesel shop area. Normally, they were dark on trailing units of a consist.