I thought most Teeside freight was going south though, and Leamside won't help with that.
The Leamside line would help relieve the already congested ECML by offering an alternative route in general. Slow trains previously routed via the ECML, could be routed via Leamside, Stillington and Teeside, for example. The Stillington line sees regular freight traffic, which itself suggests one of two things;
1. There is freight moving to/from Teesside to/from the north
2. Teesside lines are being used as relief lines for the congested ECML lines.
Bearing in mind the fact that traffic at Teesside ports is bucking a regional trend and growing, the future demand for extra train paths to ALL parts of the network will be an important consideration imo.
perhaps a Newcastle-Darlington round trip?
I personally dont think so. Running local passenger trains down the leamside line into Darlington will put EXTRA strain on the ECML, especially so considering the 70mph max speed of traction likely to be used. Also it would mean taking up valuable paths at Darlington itself, which has only two main through platforms and no northern bays (long been removed and turned into a car park). There used to be a regular Newcastle-Darlington-Teesside service operated by sprinters/pacers, routed via the ECML and the Tees Valley, but these took up valuable paths and as a result weren't very frequent.
There is also the question of 'need' and demand. Is there enough from what is essentially an ECML feeder route to warrant the cost of reopening? I would doubt it.
Rather than look for reasons to open a line like the Leamside line (which will require a large amount of investment no matter which way you look at it) the question of reopening should be borne out of need, not the other way round. Ultimately, to get the funding it requires, it will also need to fulfill or facilitate long term European and national development goals.
It is in my opinion that the currently congested ECML could be a major obstacle in the future economic growth of the region. I personally think the reopening of the Leamside line is possible one solution. Then again looking at recent economic events, who knows.......
Currently, a direct link between the two major conurbations (Newcastle and Teesside) is very poor. An hourly service via the Durham coastline takes well over 90 minutes?. Regular bus services take twice as long. I think the Leamside line could provide a link between the two cities in under one hour, which can compete with the A19/ A1 in rush hour.