But we aren't talking about the minutiae of signalling specifics. Some days you are literally staring out the window looking at the section ahead with nothing ahead of you.
Not talking about approach control, checking down, shared overlaps, double blocking, ARS, SARS, TRTS or ETC.
There are places where you can see multiple sections ahead that are "clear" but that Red just glares at you in the distance. If I get checked to Red randomly towards "clear" sections; my brain heads towards passing it at danger (typically track circuit faults). However, when it pings to green...
One of my favourites was a Signaler coming back to me admitting they were on an "Emergency PNB".
While I accept that generally it is impossible, there are many cases on routes I sign that you can see the entire section ahead of you and a good number where you can see the one after too.
But what you cannot see visually is what the signalling interlocking system knows or does not know. Or what parts of the system are not working. Or what the signaller knows.
It may be that the ARS or signaller has not routed the signal for some reason. But it could also be any number of other things.
And yes, the signaller needing to go to the toilet, but forgetting to route a signal because it's been busy is a thing. I've seen this myself when I've been on an operating floor. SPT/GSM-R ringing...
There could be a failure of part of the signalling system.
Some examples that have occurred in the past include a yellow aspect in the red signal you have been stopped at was defective or a red aspect in one of the signals ahead of you being defective. The workaround being to hold trains at a red aspect until the sections ahead are clear so that the signaller can give you a run of green aspects.
In the past, there has been a cable fault, I could give the signaller only the red and green aspects in a signal. The yellow and top yellow aspects were defective, if the signal had tried to show either, it would have gone 'black' (unlit).
Yes, if you can see greens ahead of the red signal, those examples may not be applicable. But the signalling system is complex, and there are many different possible failures.
S&T may be working on the signalling system, they have been given an occupation, but the work has taken longer than expected. Hence the signaller has to hold the protecting signal at red. Once the S&T have given the occupation back, the signaller can route/clear the signal, which changes to green...
There are various other faults or reasons.
Generally speaking, most signallers do not deliberately let trains slow or stop at red signals for no good reason. The train reporting systems on track circuit (axle counter) block areas can tell how long a section is occupied for (time) and if it's a controlled signal, if it is suppose to be showing a red or proceed aspect. I'm sure I don't need to explain any further.