It was built in competition with the Severn tunnel by a different railway company, and opened in 1879 (years before the tunnel), but was seriously damaged in 1960 when struck by two barges. Two of the girder spans fell into the river, and the bridge was abandoned, although it wasn't demolished until 1969.
It included a swing bridge over the canal at the Sharpness end and a brick arched viaduct at the Lydney end, with what looks like 17-18 arched spans, and two larger ones over the shipping channel. At the Lydney end, a line branched off to the west of the mainline before curving east and presumably crossing the mainline on the brick viaduct before reaching the girder section of the bridge. This line is now dismantled, although according to the OS maps, the branch line leading to Sharpness docks on the other side is intact, with only the southern half of the triangular junction with the mainline, and the final tight curve leading to the bridge through a deep cutting beyond the docks dismantled.
From the (blurry) aerial photos on multimap.com what looks like several shadows can be seen at the Sharpness end, and a lighter rectangle of ground standing out from the green...perhaps surviving remnants of the bridge supports and abutment? It must have been one hell of an impressive structure, both to view from below and to travel across...what a shame it was never rebuilt.
Anyway, if you feel like an adventure...
p.s. here's some pictures:
http://www.sungreen.co.uk/Lydney/SevernRailBridge.htm
http://www.sungreen.co.uk/Lydney/SevernRailBridge2.htm
http://www.sungreen.co.uk/Lydney/SevernRailBridge3.htm
