I've done some searching but came up with precious little info about the use of a Mars light during operation and how it was regulated by rules. Perhaps some of the trainmen here can discuss which circumstances would require (or not) a Mars light's use.
The only reference I found was specifically for the Great Northern:
trying to find out more about the operation of most lights, i have come to the conclusion that practically nothing was universally accepted practice and to find out anything you had to reference the particular railroad's rules & regulations. when i asked about NKP practices recently, i'm sure the answer regarding Mars light operation was, 'whenever the headlight was on and the engine was moving'. as far as i know, there was no "emergency red' setting on the NKP light.
i have come to the conclusion that practically nothing was universally accepted practice and to find out anything you had to reference the particular railroad's rules & regulations.
You're absolutely correct.
Santa Fe had a unique requirement for the red oscillating headlight. (Santa Fe only equipped passenger diesels with oscillating headlights.)
When a train encountered a Stop and Proceed signal (red aspect with number plate on the mast), on single track, it could, after stopping (and waiting 5 minutes if the Engineer could not see the track all the way to the next signal) proceed at Restricted Speed and engines so equipped were required to display a red gyrating headlight until the next signal was passed.
Rules required that the white gyrating headlight be displayed when moving on a main track at night until the late 1950's, at which time "at night" was removed and it was required day and night, when moving on a main track. Another requirement was added, to require use of it when approaching crossings. That included auxiliary tracks (sidings, yards, spurs) where the train was not on a main track, but only if approaching a crossing.
I once had a head-end Brakeman who was prone to motion sickness and he asked me to turn off thewhiteMars light while going through a canyon, which I did, although not exactly in compliance with the rule.
I find this interesting information about the mars light. This brings me to ask the question: Did railroads have rules about the yellow strobe lights on the top of the engine cabs? To my knowledge the Santa Fe and Frisco had strobe lights.
Here's a couple of things I found on the SAL/ACL forum:
ACL - The rule book (Rule 17-A) required the use of the Mars light when ever the train was moving forward at night. It was to be turned off when approachin terminals, junctions, meeting points, and stations where stops were to be made. Also, it was to be turned off when ever the rules required that the main headlight be dimmed.
Before there were Ditch lights, there were Mars Lights During the spring of 1936, an astonishing sight stopped motorists and attracted crowds of curious spectators to highway overpasses along the Chicago & North Western line from Chicago to Minneapolis. The engine was one of the railroad's rebuilt Pacific-type set aside for service on the famous "400s," but the oscillating blue light flashing from the top of the smokebox was definitely something new. The light was the brainchild of a Chicago city fireman, Jerry Kennelly, whose encounters with oncoming street traffic during emergency runs led him to tinker with various warning-light devices. The most effective, he discovered, swept the path in a horizontal figure-8 motion that caught the attention of motorists both in front and to the sides of the truck. But it wasn't until Chicago candy magnate Frank Mars and his wife, Ethel took an interest that Kennelly's invention became reality. Mars offered the inventor use of the candy company's machine shop to turn out prototypes, and after Mars' death, his widow continued financial support of the project. In return, Kennelly assigned patent rights to the Mars Light Co. Working with a group of Chicago policemen, Kennelly developed the device further, offering it to railroads for use at highway crossings. Finally, it caught the attention of Chicago & North Western's chief safety officer; he agreed to mount a Mars light on engine #2908, one of the four E-2's assigned to the new "400" service. Additional tests were conducted with a Mars light on a J-class 2-8-2 that shuttled back and forth on the Orchard track in Proviso Yard. On one of its runs to Milwaukee, the 2908 struck a large bird, shattering the blue lens. A clear lens was located in Milwaukee, and, on the return trip to Chicago, it was discovered that the white light was even more effective in catching attention. Gyrating in a horizontal figure-8 that was 800 feet in diameter, 1000 feet down the track, the new Mars oscillating headlight was adopted by C&NW for its steam-powered engines and, despite the initial indifference of EMD officials, by the Rock Island for use on its first passenger diesels. Other railroads soon followed. (from Chicago & North Western Vol. 1, by Lloyd A. Keyser, Morning Sun Books, 1997. Thanks to Charlie Willer, Heartland Rails, Ft. Wayne, IN)
From what I found out a while back, Seaboard used them for emergency situations, but don't know what defined an "emergency". Also, some of Seaboard's E7s had red lenses, others had clear.
If we want to know every instruction regarding the use of oscillating headlights, we'll need to have a rule book-contemporary copy of the specific railroad's bulletins (which were variously called Superintendent Bulletins, General Orders, General Instructions, etc.) which could modify or supplant that rule book. It is likely that these publications also contained some instructions for use of oscillating headlights, especially on railroads that used consolodated rule books which were adopted by more than one railroad.
Add these original owners: MP (passenger; also a small number of SD40's bought for pool service with D&RGW) T&P (passenger, freight, hood units) Union Pacific until 1955 (passenger, freight, GP7, SD7; not on E9's built 1955 and later, or on GP9 and later hood units) Akron Canton & Youngstown Denver & Rio Grande Western (passenger, freight) Burlington Route (passenger, freight F3 & F7, hood units up to BN merger; added to steam 4-6-4, 4-8-4, 2-10-4, FT's, F2's, and EMC shovel-nose Zephyr power cars) C&NW/Omaha Road (passenger, freight, early BLW/F-M road switchers, later all removed; also some steam passenger engines) Southern (passenger DL-109's, E-units and PA3's, later removed from E-units) Northern Pacific (passenger, freight) Nashville Chattanooga & St.Louis (passenger, freight, hood units) Louisville & Nashville (passenger, freight, hood units including RS3's) Monon (passenger, freight, second generation hood units) M-K-T (passenger, including FP7; also hood units equipped for passenger service) Frisco (passenger E-units; later added to freight cab units) Pennsylvania (2 Baldwin passenger cab units, and 2[?] E8's) Atlantic Coast Line (passenger, freight, hood units) Seaboard Air Line (passenger) Canadian Pacific (RDC's; also, later, FP7/FP9, on roof, angled upward) Canadian National (RDC's) Great Northern (passenger) Milwaukee Road (passenger, freight F7's & F9's [but not FT's], some steam engines) Rock Island (DL-109's, E-units, F2's, AB6's; later added to EMC TA's and 1 or more [but not all] FP7's) Florida East Coast (passenger, freight, hood units) Western Pacific (passenger, RDC's, second-generation hood units) Spokane, Portland & Seattle (passenger E7 & F3's) Wabash (passenger, freight, first-generation hood units) Gulf Mobile & Ohio (PA1's, Baldwin passenger units; former Alton E7's not equipped) Soo Line/Wisconsin Central (passenger, freight, GP7, GP9) Chesapeake & Ohio including Pere Marquette (passenger, some GP7's (removed from GP7's) Delaware & Hudson (PA's, second-generation hood units) Erie-Lackawanna (3 units involved in trade with D&H? - needs verification) Green Bay & Western (FA1's) Tennessee Central (FA1's) Amtrak (SDP40F's, P30CH's, GE electrics)
Chicago Great Western (passenger, freight, GP7's, second-generation hood units)
Nevada Northern (1 SD7, later removed)
Most northeast carriers declined to equip their engines with Mars lights. It's just a general observation, but most of the carriers which used them ran in parts of the country with wide-open landscape and/or many road crossings with only passive protection (crossbuck signs) as opposed to the many grade-separated crossings or crossing gates of the northeast. D&RGW notably used them also for observation, as it has miles of track in narrow, deep canyons.