I can't really answer the OP other than it is most likely the way the figures are calculated which may have changed rather than the figures themselves. I've no doubt the figures are produced by computer modelling rather than people on the ground counting. If there is a flow of passengers from the Weymouth route to Reading and Paddington there could be some reason why the model has decided that the interchanges are at Castle Cary rather than somewhere else hitherto.
I am 99.9% sure it is not by people counting on the ground! there may be some flow towards Reading and back, but my hunch is the main interchange flow is to/from the west of England to Weymouth/Bournemouth.
I do remember BR improving the services to and from Castle Cary in the 1970s and early 1980s when Parkway stations were all the rage, and at the time services on the SW route from Exeter were slow, infrequent and (most importantly) unreliable. The improved services were very successful in attracting people from the Yeovil area who found driving to Castle Cary far more agreeable than using the other route. ....
Now you mention it, I have vague memories of reading some such in Modern Railways of the day. (Or perhaps it was much later, when it was reporting that NSE/SWT were winning passengers back to the Waterloo route.)
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This morning, I did a quick count.
8 people got off the Weymouth train. 4 of those got on the Paddington Train. They were the "regulars" that I recognise as doing that normally.
That's very interesting. Not so much that four people change into the PAD train, but that four people actually want to go to Castle Cary at 06.45 in the morning! (I suppose some could be waiting for the 07.14 to Weymouth?)
Do you normally notice the same four alighting at Westbury?
It's also interesting because yesterday (09/01/17), your Exeter-PAD train was about 18 late. Instead of allowing the Weymouth -Bristol to go forward (and, I'd guess, allow the PAD train to overtake it at Frome), they held it until after the express had gone. This made the local 15 late away from CLC and ultimately, 24L into Bristol Parkway.
http://www.realtimetrains.co.uk/train/C20522/2017/01/09/advanced
(I suppose, however, that it's possible that they could not loop the local at Frome due to capacity limitations.)
I live in Yeovil and the above is what I do. Despite its a 20 minute or so drive to Castle Cary.
1. The Journey once I have got on the Paddington Train is much faster
2. It doesnt involve changing trains to get to Reading, which is what I have to do if I go from Yeovil Junction as I have to change at Basingstoke.
I know I could go from Pen Mill or Yeovil Junction now that South West trains are running some new services., but from where my house is both of these are the wrong side of Yeovil, especially Yeovil Junction. Which is why I just drive straight to Castle Cary also the car park charges at Castle Cary are less than they are at Yeovil Junction, probably cancelled out by the fuel costs to drive to Cary though...
Makes perfect sense, and works if your job is in Reading. You just need to make sure the company never thinks of relocating to Newbury or, worse still, the horrors of Hungerford
I do go from Yeovil Junction but only when Glastonbury Festival is on, as Castle Cary is really busy during that period.
Which is kind of ironic - the principal event which drives passenger usage up at Castle Cary drives you away
Thanks for your reports, fascinating.
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I live 2 miles from Castle Cary station and about 8 miles from Templecombe. I often find the SWT service more convenient given the hourly frequency they offer. It also depends where I'm going - Olympia and Earls Court via Clapham Junction is simple, Excel is a short jump from Waterloo. It's also cheaper than Castle Cary unless you get a good deal on an advance or resort to some pretty complex split ticketing!
I suspect even people in Exeter do the same, again, depending on where they live, where they are heading in London, not to forget their pockets.