The track infrastructure is already in place on the mainline from Newcastle to Palmersville, Northumberland Park and onwards towards Backworth and beyond, albeit single line after diverging from the ECML.
I think a route to Doxford International has also been under investigation, possibly along the alignment of Durham Road where the tram tracks once were. The line to Ryhope and Seaham already exists too, but there are fears that extending the metro along the Durham Coast will lead to further reduction in frequency in the Northern Rail service. Nevertheless, I think there maybe something in the new Tees Valley Metro, would is planned to go as far north as Hartlepool, maybe this could merge with an extended Tyne and Wear Metro, completely replacing the need for the Northern Rail link. I think all this would be years and years down the line though if it ever happened.
I think the track on the Leamside line has or is being removed due to metal thefts, but apparently engineers at Network Rail have claimed that the track would have needed replacing anyway if the line ever re-opened. From what I've read in the local press, Durham County Council are really pushing for this line to be re-opened. Stations like Sherburn could even be re-opened -
Then http://www.flickr.com/photos/pinzac55/3610804093/ Pinzac55
Now http://www.flickr.com/photos/67570349@N02/8083447500/ Jamie M
It's a disgrace the line was closed, it would be perfect for easing pressure and increasing capacity on the ECML. Jamie M has a photo here in the final years - http://www.flickr.com/photos/67570349@N02/8229876419/ You can see the severed line to the right that went to South Hylton and Sunderland. I'd love to see trains crossing the stunning Victoria Bridge Viaduct over the River Wear at Sunderland in my lifetime - http://www.flickr.com/photos/pinzac55/2818655764/ (pinzac55) On a related note, I think there's a line at Stillington that is being penned for re-opening, but I don't know too much about it.
I'm sure there actually used to be Red line at some time. I think it was the short peak-time only trips between Pelaw and Benton, which were replaced by extra journeys on Yellow line instead.
South Shields station looked magnificent!
Bit of drama on the metro this morning. Cab door of 4060 fell off it's hinges, not the one the driver was in. We were all told to move to the rear car (4054) which ended up packed. Workmen began fixing the problem at South Gosforth, but I don't think the train was taken out of service, just one-car operational.