From Southern Pacific Lines Coast Line Division “The Route of the Octopus” website:
http://modelingthesp.com/Operations/Train_Indicators.htmlIndicators
Train indicators displayed (usually on the lead engine) the authority under which the train operated. That authority was either a *schedule* in the Employe (sic) Timetable, or *movement* as an *extra train* authorized by *train order*, in the case of which the indicators would display "*X*####" (generally the number of the lead locomotive of the engine consist). This practice was
terminated on July 1, 1967, after which time the indicator became simply a number board indicating the number of the locomotive to which it was attached. The exception to this change was the Peninsula commute service, which continued to carry train numbers in the indicators.
Pre-1951
On the SP, indicators were the changeable number boards on locomotives. Rule 21C covers use of indicators. Schedule trains not displaying signals for a following section will indicate the schedule number only.
An an example is shown in the 1943 rulebook where a indicator has the number 2 in the indicator and the note that the last section would use only the number 2. This is a good example of how rules varied from railroad to railroad in the years before unified rules.
Probably the last section was still supposed to carry its section number in the indicators. The 1951 book is different.
Recall that until at least 1943, SP still used green flags; all sections except the last were supposed to carry green flags, in daytime. So any pre-1943 pic of a train carrying a section number in the indicators, in daylight, with no green flags-- is evidence that at that time the last section did show its section number.
Post 1950
After 1950 or thereabouts if SP train 804 had three sections, the three engines would carry "1-804", "2-804" and "804" (rather than "3-804") in the numberboards. The 1951 and 1960 rulebooks say that "804" in the numberboards denotes the last (where "last" might mean "only") section of 804.
It was a 1960 rulebook that had rule 21C about the last section running with only the schedule number in the train indicator. Remember, Sections were run 1st 2nd 3rd, etc. First section would run before Second before Third...and the last section would not have a section number prefacing the train number.
