As a Tenby exile, this does all seem like a bit of a red herring to me. When I lived in London in the in the late 1990s, I once got the HST to Penally to visit my parents. From Carmarthen down it was such a faff, as most of the platforms are way too short, meaning passengers having to move along the train to exit; it also seemed to be even slower than the usual Sprinters.
As has been alluded to in earlier posts, what might increase rail patronage - and make a real difference to locals as well - would be for the frequency to be increased, and for there to be earlier trains on a Sunday for people making long homeward journeys. I'm sure most passengers who arrive at Swansea or Carmarthen on a HST or 800 would be quite happy to change to a decent DMU on a cross-platform, easy change.
I also agree with the comments that the car becomes the suitcase for anything other than daytrips. That's the same in any area. If I'm taking the family to Scarborough for the day from Batley I'll take the train, but if going for a more than a single overnight then the car wins. This highlights the desire for sensible and regular direct DMUs from Newport west to Tenby and Pembroke, which would be good for both towns, allowing people to make easy daytrips and also alleviating the parking problems that Tenby has always struggled with.
So, if 800s will go through the tunnel - fine. But don't spend money on it, which could be better focused on improving the exisiting services.
