Yes, I knew about the Marsden line - in the rattler (or whatever it is called these days) on Shields sea front, there is some similar information about that line.

Just found this site too https://sites.google.com/site/waggonways/history-durham

The line I knew least about (wasnt mentioned on the site above) was the line from Rainton Bridge to Ryhope.
It made its way up to the Copt Hill at Houghton at the top of Hall Lane Estate and then cut across the top of Warden Law, dropping down behind the go-kart track and Burdon etc towards where Ryhope Engine Museum is and ending at Seaham (or via Doxford Park and Silksworth depending where you read about it, ending at Sunderland).
Personally I would have thought Seaham as Lord Londonderry was involved in both the pits in the area and the Harbour at Seaham). One site states both were used, via Silksworth initially and then routed towards Seaham.

Getting back to the S&T line and Consett Steel Works, I often thought about the Derwent Valley line as an alternative too - maybe because it dropped goods at Dunston rather than further down the Tyne like the S&T did, but again, Im surprised the steel works didnt use that either.