I’m not quite sure what you mean- do you mean the RA and CD signs that come up or the new indicators on the Arosa line perhaps?
OFF meant the next signal was a proceed, ON meant it was red. There was also the "RA" light, which meant "Right Away", an old term for "OK to leave the station driver". Usually placed on a curved platform and I say in the past tense, probably still the same now.
The indicators serve a number of purposes: OFF indicators are used for dispatchers and guards when they're unable to see the signal for their dispatch purposes. CD/RA is mostly used for Driver Only Operated services, and is used to tell the driver to close the doors and gives them the right away (permission to depart), however there are some guarded services that make use of these too.
It is still in use, but why? My guess is so that there isn't an accidental SPAD after the guard gives the double buzz.
When you dispatch a passenger train there are two dispatch checks. The first check is to make sure the signal is green, everyone is clear of the doors and nothing is attached to the train e.g. ramps hoses etc. The dispatcher does this and signals to the guard with a raised baton. If the service is DOO (Driver Only Operation) then the dispatcher will do this by illuminating the ‘CD’ light for the driver. Once the door is closed the second check commences, again the dispatcher checks the signal, checks nothing is obstructing the sides of the train e.g. people caught in the doors or leaning against the windows saying goodbye etc. Once satisfied they’ll give the right away to the guard or if it’s DOO they will illuminate the ‘RA’ light for the driver which is the same as receiving ‘two buzzes’ from a guard. The main signal only shows whether the route ahead is clear, it doesn't show the train is safe to depart as it cannot take into account what’s happening on the platform. If there is no dispatcher present then the Guard does this all on their own. The dispatcher, Guard and Driver all check the signal at every stage of this process. It’s hoped that with as many people as possible checking that a SPAD is very unlikely although they still happen. The OFF indicator or repeater is so dispatchers and guards who may be positioned towards the rear of the train during this dispatch process can see whether the signal is at danger or clear. I hope that clears it up a little for you.
It's the responsibility of the guard and dispatcher to also check the signal is showing a proceed aspect. If they're unable to see the signal then an off indicator should be provided (although isn't always). A RA indicator is linked to the signalling system, so afaik won't clear on a red. This further reduces the risk of a SPAD. It's why the RA indicator is used on P1-3 at Reading for guarded services, as a SPAD would be extremely high risk there.
I don’t really understand the question to be honest. OFF indicators are mainly for the benefit of dispatcher and guards especially on curved platforms where the starting signal is not visible. So the OFF indicator let’s them know that the signal is clear and it is ok to dispatch. If you’re a driver, then you don’t usually need to look at the OFF indicator as you will be able to see the starting signal. If you’re unable to see the starting signal such as when you’re a big, curved terminal station and you’re a short train (e.g. a turbo at London Paddington) then the OFF would also let the driver know that the signal is clear. A banner repeater at stations also serves the same purpose. E.g. if you’re a dispatcher or guard standing here, you cannot see the starting signal. But the Banner Repeater which you can just make out displays whether the signal is clear or not.
a.paice says it better than I did but yeah, there were some cases of "Starting against a red signal" and SPADding in the late '90s, so they brought in the DRA's in cabs of units. Not on our freight locos, but as freight drivers, we weren't as much in contact with the danger of starting against a red out of a platform.
I think you're making a link between two systems which don't necessarily cover the same processes. Light signals show the state of the track ahead, whether it's blocked, clear or the driver should proceed at caution The signals used to dispatch a train in platform are slightly concerned with the line ahead, but are more concerned that the platform and train are safe, so the train can depart the platform. As the people above have stated whether the train area is clear of passengers, ramps etc So these signals are important to allow for checks to be undertaken that a driver may not be able to, and certainly where the platform staff have the duties to ensure the safety of the platform before the train departs
Well, the driver has systems in place to prevent this happening. If they were pulling into a platform on a through service and the light on incoming was anything bar green they would get a warning, when they stopped they would have set DRA which stops the train applying power This is also set on shut down cabs so even if the driver was getting into the cab as a terminus and looking to set off without the signal being clear he shouldn't have cleared the DRA without first checking the signal So yes, the platform staff check the signal to ensure it's not at danger but the driver would check these anyway and have their own systems in place
while your question has been answered just wanted to highlight it’s everyone involved in dispatch’s responsibility to check the signals before a train is moved, hence why there’s different signs for different members of crew like guards and dispatchers. if you want, I’ll find the rulebook extract of the different dispatch signals.