If you wade through the convoluted history of the Severn & Wye (under whichever name) on Wikipedia you find this:
The Lydbrook Line
Forest iron ore was in demand at iron works in South Wales, and the Lydbrook branch was conceived to allow a more direct route to iron works at the Heads of the Valleys, in particular at Ebbw Vale and Dowlais. The line was to connect at Lydbrook Junction with the Ross and Monmouth Railway, then being planned. The S&WR Lydbrook branch was authorised on 12 May 1870. There was to be a triangular junction at Lydbrook enabling direct running towards Ross-on-Wye, but this was never built.
The original line went to Lydbrook, which is not where the junction is (it's in Stowfield). It was built (as a plateway) to take coal and perhaps iron ore from Lydbrook to Lydney. The 1879 extended line was for traffic the other way, and the never-built chord would have served customers to the north. Presumably these turned out to be an illusion.
Passenger traffic was always secondary, limited, and local. I wonder if there ever were through passenger trains on the line.
