As has been pointed out, Durham-Sunderland would essentially require the construction of a new line across the Wear Valley, or a new terminus in East Durham. This isn't justifiable.
Leamside has more scope. The ECML capacity review is quite clear on the advantages:
And later on
The conclusion at the moment in that document is that under current projections, the case for reopening is marginal. However, if extra capacity is required for services that will run to the Leeds branch of HS2, this would strengthen the business case.
Leamside has more scope. The ECML capacity review is quite clear on the advantages:
reopening of the Leamside line between Tursdale Junction and Pelaw would be required if there is any increase in requirement for freight and passenger paths north of Northallerton
And later on
Re-opening the Leamside Line would be the best solution to provide additional capacity
The conclusion at the moment in that document is that under current projections, the case for reopening is marginal. However, if extra capacity is required for services that will run to the Leeds branch of HS2, this would strengthen the business case.