Request stops seem a mad anachronism to me, to be honest. I also can't really see the benefit, apart from the odd train that is 2 or 3 min late being able to make it back to "on time" if it runs through a bunch of them.
But I could be wrong, so let's take a hypothetical simple example - maybe the way I'm thinking about this is all wrong in which case I am happy to be schooled by those more in the know!
We have a branch line that comes off the mainline, has 2 request stops, a mandated stop, 3 request stops and then the terminus.
ML - R1 - R2 - MS - R3 - R4 - R5 - TR
Let us also assume (?) that the driver has to slow down to check for passengers but not stop in the event that he passes through a request stop (assuming no one is alighting), which is my understanding based on the thread. Let us say this costs 30 seconds rather than 2 minutes for a full station stop.
From the above it also sounds like if we have 5 request stops we can expect to stop at 2 or 3 of them every journey. So the timetable is set that between the mainline and the mandated stop it will stop once, and between the mandated stop and the terminus it will stop twice.
If the train doesn't stop between the mainline and the mandated stop it presents early and has to wait to depart on time.
If it has to stop at both request stops it arrives late. Assuming it then stops at another couple as expected It will then arrive late at the terminus where its layover would provide the buffer to soak up the delay anyway.
All we've done is create an opportunity to maybe shave a minute or so of time off the whole branch timetable. Why or for what?
- we have made the journey more intimidating for passengers "be there AT LEAST 5 minutes early and push a button, be paying attention, stick your hand out, be clear, it's YOUR FAULT if the train doesn't stop for you"
- Added extra duties to the guard "where are you getting off? Don't forget it's the next stop, I'd better remind the driver"
- added extra uncertainty for the driver "what if someone is on the platform?"
All of this for absolutely negligible timetable gains.
The joy of the railway is it's reliability and permanence. There is a station. Trains run every 30 / 60 / 120 minutes. They stop at the same places. There is a system you can rely on.
One of the reasons people prefer trains to buses is that they aren't caught up in congestion, snarl ups, etc. they tend to arrive broadly when expected and stop in the same places. They aren't expected to be 30 seconds or 3 or 15 minutes different every day based on external factors. Obviously they actually are, but they have good Realtime Information to overcome that, but as a concept they are much more reliable in terms of presentation and journey time than buses. So... what benefit does binning that off bring?
Just sling in the extra few minutes to stop properly at the locations, remove the uncertainty for everyone and... i don't see a real meaningful downside?