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Odd one this morning. The 0629 Swansea to Paddington was announced as stopping additionally at Severn Tunnel Junction (I got off at Cardiff; so no idea whether it actually happened, or was just some sort of glitch). I can't see any evidence of any sort of disruption. Anyone got any idea what this was about?

Semi-educated guess - were there any short forms around on Cardiff via Bristol to Taunton or via Westbury services?    If so, extra stop could be a crowd-buster.    Sometimes done at Oldfield Park and Keynsham.

Edit - lots of short forms showing at present on Cardiff - Portsmouth so likely they were also short formed in the morning peak.

This happens fairly often with this train if 0730 Cardiff to Portsmouth, a very busy service between Cardiff Cen and Bristol TM,  is short formed with only 3 (or even 2) carriages instead of the booked 5.      If short formed, passengers at Severn Tunnel Jcn are unable to board.

I think it was 3 times last week, and again today...

The London train has a good connection into the 0812 BPW - WSM, so not too bad an option for S Wales to Filton Abbey Wood or Temple Meads.  Would help if there was an announcement at Newport that passengers awaiting the Portsmouth train might like to catch the London train instead: only when the Holyhead train has left P4 does the train indicator board say how many carriages will be on the Portsmouth train, by which time the London train is already on the approach to P3.

Indeed - quite useful when it is done too! Also much easier to do now we have IET's where the TM doesn't have to walk up and down the train to make sure all the doors are shut!
Though would be nice for the decision making involved in this kind of thing to be more transparent to passengers.
Talking of Oldfield Park and Keynsham, my local stations, there have been times where it seemed to me (based on NRE, RTT etc) that a London service should be stopped but it wasn't, and other times where it hasn't made much sense for a service to be stopped when it has!

Stopping again this morning - and JourneyCheck confirms why

The STJ stop on the 0629 Swansea to Paddington has continued on a fairly regular basis.  One or more short-forms on the 0730, 0800 and 0830 Cardiff departures to Bristol TM and beyond has been pretty normal for the last few months.

However, it will be interesting to see whether GWR continue with this practice from next week.  This Swansea to London train becomes one of the fast ones that runs non-stop from Bristol Parkway to London and leapfrogs the preceding Cardiff to London service en route.

Yes - the 0628 ex Swansea does still make the STJ stop when the Portsmouth train is short formed.  Happened today for what I think was the first time since the new timetable started.  The London train was half an hour late though - so it actually helped to fill in for the cancelled 0800 Cardiff to Temple Meads.  Not a good Monday morning.

Now that there are 'stopping' and 'fast' trains from South Wales to London, why shouldn't Severn Tunnel Junction have at least one daily service each way to the capital?  At the moment the available routes with one change are via Newport or Bristol Temple Meads, adding half an hour or more.

Proposed by Grand Union Trains with some discussion at http://www.passenger.chat/21288

On many lines, London / long distance expresses call at the more major stations with local trains between them. In a very few cases, those local trains do NOT connect the more major stations at each end, and the Newport to Bristol Parkway stretch is a case in point;  there is "nothing" (*) from Newport via intermediate stations to Bristol Parkway and that means that Severn Tunnel Junction, Pilning and Patchway don't even have a service to London via a connection at the next station towards London. 

(*) - I have quoted "nothing" as there is actually one though train per day from Severn Tunnel Junction and Patchway to Bristol Parkway. It's the Cardiff to Manchester Cross Country service and it runs via Bristol Temple Meads!

Maybe if TfW eventually run Ebbw Vale to Newport services, these could be extended to Bristol Parkway, calling all stations.  This would also make sense if a new station at Magor ever comes about - otherwise I can see that getting just the Cardiff to Cheltenham trains stopping there.

Makes huge sense

Blimey yes. And a connection there with the Henbury Loop would open up a further world of opportunities.

It would also make the case for reopening the loop rather than the spur yet more compelling.