The red oscillating light as used on the SP is to indicate the train's brakes have gone into emergency which could be caused by parted airhoses on cars that may be derailed and foul adjacent tracks. When approaching a train on another track and the UDE (UnDesired Emergency) light operating, your are supposed to slow down to a speed that allows you to stop short of anything fouling your track. This could be derailed equipment or shifted loads.
As to why some F units have the oscillating lights in the upper housing and the steady, normal headlight in the lower housing, this is strictly a guess and I could very wrong. The steady headlight being placed lower, can be aimed to provide better illumination of the track and roadbed, much like today's ditch lights while, the lights in the upper housing provide the attention-getting illumination.
Also, if the mechanism for the Mars or whatever gyrating headlight that was used, filled that housing SP used on its units equipped with Mars lights, you wouldn't want all of that mounted on a door. It would completely nullify the door!
I always wondered why some F units had the “upper” headlight red and the “lower“ headlight white!
I’ve seen photos of Athearn Genesis F3’s and F7’s of UP, MILW, and CNW showing this feature. Other roads might have had this feature as well. Guess the red light would act as a position light to indicate oncoming trains on a double track mainline the lead F3/F7 displaying a “red light“ on the front told engineers that train was stalled, a mechanical problem had occurred, or the lead engine was having issues.
Mars lights are effective warning devices. The light moves in a elliptical or circular pattern, causing the beam to ”bounce” off the ground to the left/right/up and down This helps identify the train at grade crossings, stations, yards, and rural areas where it’s “pitch black” under the cover of darkness
John, I've been passing this thread since you posted it and tonight it jumped out at me. I grew up a stone's throw from the Milwaukee Road main line between Milwaukee and the Twin Cities and remember the diesels with their Mars lights. Later, when I was a teenager, we lived six blocks from the Soo Line main, with their beautiful F7s pulling a northbound passenger train into town, swapping out head-end cars, then proceeding north again. That train came through at about eight in the evening, so the Mars light swept thither and yon as it came to the 4-track grade crossing down the street from us. There was absolutely no doubt there was a train coming with that infinity pattern showing the way! (Two buddies and I rode the cab, one night, with permission one of 'em had obtained and it was strange, like standing in the corner of a second story room and having it move, swaying and clanking over the switch frogs.)
Deano
RR_Mel The Southern Pacific used the MARS lights on almost every road diesel locomotive including the Geeps and SDs. The PAs, F and E series locomotives had the MARS light on top and the Headlight below mounted in the access door. They retro fitted many of their Geeps and SDs, they mounted them above the hood. My main go-to site for SP info is ESPEE Modelers Archive. http://espee.railfan.net/espee.html Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California I'm beginning to realize that aging is not for wimps.
The Southern Pacific used the MARS lights on almost every road diesel locomotive including the Geeps and SDs. The PAs, F and E series locomotives had the MARS light on top and the Headlight below mounted in the access door.
Yesterday I posted under the DCC/Electronics forum a question on an LED upgrade to a Gensis F3A. Included was an indication that I'd like to have a Mars or Gyralight function on one of the two headlights. That posting might have been premature.
Initial, surface-level research this morning is rather inconclusive. Most of the forum discussions regarding Southern Pacific's use of either type mentions their first use in GS series locos and perhaps freight locos in Daylight livery. There's also mention of use on some GP diesels. But were Mars or Gyralites focus ever used on F units? And, if so, was the oscillating one the upper?
I wouldn't classify myself as a rivet-counter. But I think something as obvious as this really should be prototypical.
John