But is it fair to say a fair number of trains terminate at Avonmouth rather than continue to Severn Beach?
It is indeed fair to say that, because it's true! There's a couple of early morning trains to SVB, after which they go on the odd hour xx16 from
BRI» . After 1716, they get a bit random. The explanation lies in the limitations imposed by the track. It is double track through Lawrence Hill and Stapleton Road, then single from Narroways Junction. There are passing loops at Clifton Down and Avonmouth, and although there are many tracks between Avonmouth and the turn for the Henbury loop, the track to SVB is effectively single from AVM.
Pre 2008, the service was really no more than a shuttle between BRI and SVB, with a bit of infill. As it is 74 minutes plus changeover time for the round trip - say 80 minutes - that doesn't make it easy to have a clockface service. They who know much more than I do about these things worked it all out, and realised that by turning back two in three services at Avonmouth, effectively a 60 minute round trip, the third service could go on to SVB, with the whole thing run using two sets. Genius!
The Greater Bristol Metro holds the promise of making things better still. Portishead to Severn Beach should be attainable within an hour - just. So should Portishead to Bath, and joy of joys so too should Severn Beach to Bath. So a half-hourly Portishead to SVB, and a half-hourly Bath to SVB will give 4tph at each station from Severn Beach to BRI. Something I will believe when I see.