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.
