Yeah, you're right, it was "special train following" not the staff. I was trying to remember the instructions from an etched sheet of discs I bought 1990 or so.
My GA says refer to Rules and Regulations (specifically rule 150, 151...), which I don't have.
Rule 150 (a), only refers to a white tail tail disc on the rear of complete trains, or tail lights on the rear of a train in daylight. In darkness & foggy weather tail lights are to be on.
(b) refers to lights on the rear in night time operations
(c) When a train is banked in the rear a tail disc, or lights must be displayed on both Bank Engine and on rear of the last vehicle on the train.
(d) (i) Light engines on any running must carry tail disc in the rear during daylight. & lights in darkness or foggy weather except shunting engines.
(ii) when two or more engines run coupled tail disc must only be in rear of the last engine or lights in darkness and foggy weather, on the rear engine only.
(iii) An engine hauling vehicles must not carry disc or tail lights on rear of engine
These instructions are in the 1959 book of rules and regulations.
Rule 151 is pretty much a repetition of rule 150 except it applies more to the guard who must ensure the discs and lights are on his vehicle at the rear.
Special trains Rule 153 (C) Special discs & additional lights must be provided to indicate the running of special trains on the the sections and during the hours prescribed by the CTM,
i) A red disc or red flag in daylight or additional red tail lights during dark or in foggy weather carried on the centre bracket of the the last vehicle of a train or engine indicates a special train is to follow,
(ii) On single lines a white disc with black cross in daylight or foggy weather a red light must be displayed on the rear of the train or engine is due to run in the opposite direction.
This rule goes on to direct the duties of the station staff in regards to ensuring the special train is known and conveyed to those who basically need to ensure its running.
No mention of the Triangle tail discs though, but from what I can remember they were mostly found on passenger trains, I also saw red ones.
In going through these regulations, the aspect regarding marker lights in darkness and foggy weather for engines the rules applied that in daylight hours once fog had lifted the lights/dynamo had to be turned off, reasoning was to save steam and water, some inspectors were savage on that rule, and would report the crews who did not follow them.