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The FGW north downs services appear to have quite short dwell times at reading when compared to south west trains. It only takes the incoming service to be a few mins late and this has a knock on  for the outgoing service, compounded by getting stuck behind a SWT stopper.

I am guessing this is determined by rollingstock availability, or are there other factors involved?

I've always assumed it's because the FGW trains are "home", and change crews. If a train is very very late they can (sometimes) find a replacement in the depot for the new crew to take out. The SWT trains are "away", and the crew takes a break before going back with the same train.

Or is it because of the length (in minutes) of the route, and how many minutes need to be added to make the service clockface?

Take 2 services that run every 2 hours.   One has a running time of 50 minutes, operates with a single train, and it turns round at each end in about 10 minutes.    The second service has a run time of 70 minutes, requires 2 trains to operate it, and has turn arounds of 20 minutes at one end and 80 minutes at the other.

Of course this is timetabling, so lots of different factors have to be juggled into place at once. Striking a balance between reliability (recovery time) and rolling stock usage is one, what paths are possible in the overall timetable is another. The other end may be more important, as at Waterloo where platform dwell and paths are highly constrained.

In this case the two journeys are very similar in length, but the Waterloo trains do a cycle in 3:30 while the FGW ones take only 3:00. They alternate slower to Redhill with faster to Gatwick, which is an added constraint. The time at the other end is not actually any longer than at Reading, so it looks as if the crew break is not the most important factor, at least in this relatively short round trip.

If two trains per hour will have to go to Gatwick, to justify the cost of that shiny new platform 7, I wonder what they'll come up with as a pattern then. So far the effect of having P7, and putting Gatwick Express trains into P5-6, is that some FGW trains now dwell in P2 rather than using P3 and briefly lurking in a siding.

I expect the timetabling is partly designed around the previous two platform terminus, and no-one has bothered to change do any fine tuning now that there are three platforms. 

However arriving FGW services have to be organised to tend to catch up the previous SWT at Reading, and departing FGW have to run ahead of the SWT departure half an hour later, due to SWT being the all stations stopper.

As said above SWT having most of their layover at Reading compared to Waterloo is probably to allow more efficient operation at the Waterloo end, which is busier.

Then the shared part of the route's overall timings will probably be designed to give a fair run for FGW services to cross the junction at Wokingham with a few minutes leeway over the down direction SWTs that they'd cross in front of, or behind.

Paul

A fine example of long turn rounds is SWT's Portsmouth and Southsea (PMS) to Southampton Central (SOU) stopping service, which uses three units.   They get about 6 mins at SOU and about 55 minutes at PMS.  (If you catch this service you'll sometimes be held at a signal on the approach to PMS while the departing train leaves.)

Running time is about an hour, so three units have to be used.  A theoretical 2 unit service is possible with some speeding up, but there'd be no scope for timing errors over the course of a whole day, and it would also be impossible to extend the peak services to Portsmouth Harbour like they do.

Paul

If/when 2tph run to Gatwick I think there will have to be an extra unit allocated to the standard pattern for the timetable to work. At the moment the daytime service uses 6 units with fairly tight turnarounds. If the Redhill train is extended to Gatwick you'd probably need a 7th unit, but I expect you could also timetable for slightly longer turnarounds to improve reliability.

It's interesting to see another even more extreme example. Goodness - so there's a train parked up for 55 minutes out of 60!     My example was Swindon to Cheltenham Spa and Swindon to Westbury, in case anyone hadn't twigged and for years that unit sitting in the bay at Swindon, and another unit sitting in the bay at Salisbury more often than not, taunted us!

The Thames Valley stoppers have a turnaround of about 30 minutes at Reading and about 10 Minutes at Paddington