Although some of the following details may have been altered in the latter days of changes to traditional roles of 'who does / is responsible for, what', it is my understanding that OFF indicators, though perhaps most frequently found on or associated with, station platforms, can be necessary elsewhere, e.g. for shunter/guard/driver information in some sidings / yards where methods of movements and/or the siting or application of a siding(s) exit signal(s) may be difficult to see or is(are) unclear.
But where an OFF indicator
was provided in connection with train despatch from a platform, it was normally intended for guard and/or platform staff use : [ I believe that if drivers needed a repeat of a platform starting signal, it ought always to have been a Banner Repeater signal,
remembering that, at through platforms, the repeating may have also been a necessity for non-stopping trains.]
Although I wouldn't dispute most of Mark's comments, my own experience of these does not agree with his account in all respects.
In at least some places, with the early introduction of DOO working of passenger trains, the initial 'Right away' indication was merely an illuminated 'R'
[ the irretrievable 'R' as one safety-worried ops. manager disparagingly called it, because there were no CDs then, and illumination of the 'R' allowed the train's driver to close doors and the train then to leave providing only that his/her interlock light lit in the cab, and there was no way to cancel the 'R' if the staff saw anything unsafe on closure of the doors (it was only extinguished by the signal going to red), which is still the case with RAs now, but perhaps this is appropriate, as once having permission to depart drivers ought to then concentrate on signals and the line ahead, and not on whether they should watch to see if the RA is extinguished ].
Only in later days did the standard indication become RA
(actually not the first time this had been used to indicate 'Right away': e.g. until the mid-1960s, platforms at Paddington had "RA" indications in association with the still-working ex-GWR colour-light signalling).
And introduction of the CD then meant that the RA could solely mean 'safe to depart'.
In more recent years, doubtless as a safety measure, the RA may instead have to be operated by platform staff using their special key to turn, and then release, a spring-loaded key switch (the same would apply to the CDs where provided : Even the control of the OFF often became protected if still a plunger (e.g., in a locked cupboard), or was also by key switch : (Pre-1989 resignalling of Liverpool Street, it had not been unknown for experienced, observant commuters on a train whose departure was being delayed, to nip out and press the 'Train ready' plunger, presumably hoping this would hasten the start of their journey home!)
Mark Lamb wrote: " .... The RA indication illuminates when the RA plunger on the platform is depressed (signal must be off), and usually remains lit until the signal is replaced. The lighting of the RA indicator extinguishes the OFF indicator (never the two lit together). .... "
In my experience, OFFs always remain lit until the associated signal goes to Danger.
My impression is that if CD and or RA facilities are present as well as an OFF, that the OFF would stay lit while the signal is Off : I feel pretty sure that I have seen, somewhere, instances of OFF and RA lit simultaneously. I would have thought that extinguishing the OFF when an RA illuminated could cause concern to some staff that the signal had gone back to Danger, but I cannot be totally certain on this.
A footnote on modern OFFs : Where these are provided on a bi-directional platform line, it is not uncommon, in order to avoid confusion, to see that the display is OFFDN or OFFUP, indicating which direction the OFF is lit for : This is obviously particularly necessary if there is a starting signal
at both ends of the platform, and/or the OFF indicators are themselves double-sided and simultaneously show the same indication(s) 'fore and aft'.
Mark Lamb wrote: " .... I've never met CD indicators, so can only speak for the OFF and RA indicators. .... "
As touched on above, CD indicators were introduced
(indication now often displayable by the same indicator as the RA; there never being a really valid reason for CD and RA to be visible at the same time) to enable the RA to become a post-doors closure, simpler, positive indication to drivers that it was safe for the train to depart.
As to when CDs are operated, I'm unsure what current standard practice is for new installations regarding whether they can be lit with the platform signal still at Danger. In these days of extensive assessment of risks, the signal may have to be Off in case a driver somehow misread CD and took it to mean 'right away' (i.e. signal Off), but I feel sure that there are (have been?) some busy locations where, to optimise platform occupancies, CD could be operated to get doors closed before the signal cleared: The perhaps most obvious such situation would be 'a.m. peak' at busy termini where arriving passenger trains depart as empty coaching stock. A 'free' CD would then, while waiting for the departure signal to clear, allow platform staff, once sure that everyone had alighted, to use the CD to have the driver close/lock the doors ready for a prompt departure on clearance of the signal, and thus saving precious seconds in getting the platform cleared for another arrival.
So I'd suggest that Mark's sequence be modified as follows (my alterations in
red) : -
" I would expect the sequence involving CD indicators to be:
1) Train Ready to Start plunger
or key switch operated by station staff,
2) signaller receives request and clears signal
when safe and correct to do so,
and OFF indicator illuminate
s so station staff can continue with despatch,
3) CD plunger
or key switch operated by station staff
when safe to close doors,
4) CD indicator lit
allows driver
to close doors,
5)
if CD operated by key switch, station staff release switch when doors closed, CD extinguishes, and station staff confirm safe to despatch and press RA plunger
or key switch,
6)
CD (if plunger-operated?) extinguishes
, RA lights up and
if driver's in-cab interlock indication is illuminated (= all doors properly closed), train
can depart."