This is very interesting (and pertinant to what I'm asking) but I'm not sure how to interpret it - it raises more questions than it answers, but perhaps of necessity.
* Trains usually take 7, 8 or 9 minutes from Trowbridge to Westbury - with occasional ones taking 6 or 10. From Westbury to Trowbridge, the time is almost universally 5 minutes, with the very occasional 6 minutes. May I take it that the "working timetable" for a train that has 8 minutes to get from Trowbridge to Westbury really has it arriving there three minutes early?
Disclaimer: this is based on observation/research/guesswork!
Going towards Westbury: the ROTP will also has a section stating minimum timing allowances approaching key points. So approaching Westbury there is a 1 minute minimum applying to the working timetable. The public timings may add further padding, particularly when trains terminate. THe matter is confused further by pathing time - towards Westbury this might relate to platform/junction conflicts; going the other way it might relate to headways.
From Westburty: the 5/6 minute variation might come down to half minutes. Often in cases like thatn it turns out that the advertised dwell time also varies by a minute, and the variations average out over the journey, if that makes sense. From observation it seems that half minute arrivals tend to be rounded up in the public timetable.
For example:
Dep Westbury 10.00, arr Trowbridge 10.05, dep Trowbridge 10.06 might be published with those exact times.
But dep Westbury 10.00+1/2, arr Trowbridge 10.05+1/2, dep 10.06+1/2... might be advertised as dep Westbury 10.00, and arrive/depart Trowbridge both at 10.06.
In both cases I've assumed 5 minute running time, but the public times vary between 5 and 6.
* The "Rules of the plan" say 10 minutes at Westbury - I think that was what was being referred to. Is that based on the working timetable (so that a public timetable could show a 7 minute turn around for a train that is scheduled to take 8 minutes from Trowbridge) or the public timetable (in which case, the real scheduled turn around time wouldbe a minimum of 15 minutes for that train with the 10 minute schedule)?
I'm fairly certain it would be based on the working times. Otherwise many of the 'west' services have 'illegal' turnarounds! Similarly with ordinary dwell times -
HSTs▸ don't really have 0 minute stops at Swindon etc.
* What are the rules after three minimum turn arounds? An extra minute? An extra five?
That is the question that springs to mind! I suspect when the rules are vague or flexible it comes down to the discretion of some timetable person at Network Rail to agree or disagree to the times put forward by the
TOC▸ .
* Why is the Westbury "base" figure so much higher that for other places? 150s don't "stretch" to 3 times there normal length when they enter Wiltshire
so it's no longer walk for the crew, and overcrowding / delays tend to be more of an issue at Trowbridge and Bradford-on-Avon, and on trains that are NOT ending at / starting at Westbury.
As suggested, perhaps it's related to making things run on time through the Bristol pinch point... but then the same issues apply in many places. 3 is the minimum, but having a quick scan through, 10 is also common - e.g. it's the minimum turnaround for a
DMU▸ at terminating at Swindon. At many locations there are many minimum values depending on journey origin/length - see Bristol and Paddington.
I wonder if it's based partly on statistics - if a location happens to have frequent late starts on certain flows, or issues with knock-on delays, perhaps someone nudges up the minimum turnarounds.
Incidentally, the Westbury value is 10 minutes 'from Weymouth/Bristol/Salisbury' - so a Melksham line service could turn around in 3 ;-)
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