Quoting from the September 1, 1959, Southern Pacific "Rules and Regulations of the Transportation Department":

"17-D Oscillating white light on engines so equipped must be operated during stormy weather day and night, foggy weather during daylight hours only and must be operated when approaching railroad crossings at grade both day and night under all conditions.

Oscillating red light on engines so equipped shall be operated by day or night, only when the train is proceding under the provisions of Rule 510(b) or when a train has stopped, or is stopping, under cirumstances that may cause an adjacent track to be fouled, and will not in any way relieve trainmen or enginemen from compliance with Rules 99, 102, and other rules. A train or engine on the same or adjacent track must stop at once, and may proceed only after ascertaining that track is safe for passage of trains."

Rule 510(b) was an Automatic Block Signal System rule. Rules 99 and 102 were flagging and "train parted" rules, respectively.

"19-B. Oscillating red light on rear of trains so equipped must be operated continuously day and night while train is on main track except may be extinguished when train is between siding or yard track switches to permit passing of signals. Light must be extinguished when train is clear of main track. Red light shall be turned on and turned off by trainmen; or by enginemen on light engines. Display of red light does not relieve conductors or engineers from providing proper flag protection, or from complying with other rules."

Not sure when the rule was modified to require the use of the class lights displaying red on the rear of a light engine or a helper engine operating behind a caboose instead of the signal light, but it was prior to my arrival in 1971.

As info, the pressure switch in the red signal light control box mentioned in some other posts was set to drop out (thus causing the light to come on) whenever the brake pipe pressure was below 30 pounds, plus or minus.