Because, on a two track railway, a train stopping at all the "local" stations is going to get caught up somewhere, so having it terminate at Gerrards Cross allows that service to be run by fast accelerating stock with a "metro" layout and keep that separate from the longer distance services - unless you are planning on running everything as all-stops, you're going to have to draw the line somewhere.
There's a siding at the north end of Gerrards Cross (AFAICR) so why not focus the stoppers as far as there (given that they'll have the subsequent ("fast") service from Marylebone on their tail after about eight stations
I was just looking at the "unique" bit of the Chiltern network at first - i.e. the stations where they have more flexibility to change things - I thought that everything did stop at Warrick/ Solihull etc though - certainly they all should in future
1. The need to compete for London - Birmingham journey times is going to be less important in future
2. By stopping Birmingham services there, you remove the need for a "local" service on the line beyond Gerrards Cross which improves reliability. One of the problems that we have on a number of sections of lines is where we are trying to find space for a stopper for the sake of a couple of local stations, which gets in the way of longer distance services - it's not about demand for services from Saunderton to Birmingham - just a way of allowing places like Saunderton to retain a service to High Wycombe/ London etc without having to run a separate train just to tick the box of serving one station
I was suggesting having four "semi fast, longer distance" services per hour roughly every fifteen minutes from London towards Bicester, with another four paths coming out of Marylebone, made up of the half hourly Aylesbury service and a half hourly stopper towards Gerrards Cross (i.e. a second "fifteen minute" service out of Marybelbone - although obviously not stopping at the same stations, given the way that the Aylesbury line diverges away to Harrow etc. A simple fifteen minute service from London towards Bicester stopping at Gerards Cross/ Beaconsfield/ High Wycombe/ Princes Risborough would be a lot better for people in the "Chiltern" area than spending so many resources running fast trains through this part of the world for the sake of competing with Avanti/ LNW. For example...
- xx:00 - Birmingham
- xx:04 - Aylesbury
- xx:15 - Oxford
- xx:19 - Gerrards Cross
- xx:30 - Birmingham
- xx:34 - Aylesbury
- xx:45 - Oxford
- xx:49 - Gerrards Cross
(etc)
I thought that if I suggested that the "slow" services ran just
three minutes after the "semi fast" services then someone would say that the signalling didn't allow such tight margins, so I thought I'd go for a four minute gap in the hope it wouldn't get a sarcastic response...
How do you run a regular stopping service to the stations between Marylebone and Gerrards Cross though? You don't want them to just terminate at Gerrards Cross (preferring that they run further), but that just increases the chances of them being caught up, making the timetable less reliable - so a longer distance service gets caught up behind a stopper. Trying to timetable everything so that the "stoppers" have to wait in a loop near Ruslip adds on at least five minutes to journey times, which is very unattractive to passengers (seeing your service overtaken, finding that a journey that could be fifteen minutes long now takes over twenty minutes because you are sat in a loop waiting for the service that was behind you to overtake you and then the signal clears...).
The only other way to create more room for the "local" stations would be to run the Oxford and Birmingham services within a few minutes of each other, giving more space in the timetable for stoppers (but then this would mean a lopsided service for middle distance passengers and probably mean a disproportionate number of passengers on whichever of the two longer distance trains runs just ahead of the other one.
But then, if we can't serve the "local" stations at the London end of the route then maybe we just get rid. Infrequent services are acceptable in rural areas but pretty pointless within the M25 - either come up with a timetable that gives places at the London end of the route at least a half hourly service or look at closing them down (and force people onto the Underground) - either make it fit for purpose of get rid.