"Captions on two of the pics in the lower part of this page suggest the line was used to meet pleasure steamers and that it remained operational till 1975"
I can vouch for that.
As a school kid in Barry I can remember our annual school trip to Bristol Zoo, and for a change we went on the paddle steamer across to Avonmouth.
We all marched down to the train station, and boarded a special service which went thru Barry Island station, thru the tunnel, and onto the dockside, where we all embarked onto the paddle steamer.
(I get the feeling it was a few Barry schools, not just ours - High Street)
Then we dis-embarked at Avonmouth & got buses to the Zoo.
If memory serves me, we all returned by train.
Had to have been about 1965/66 ?