What would the main barriers be to splitting off from the Northallerton – Eaglescliffe line, tunnelling under Middlesbrough and the Tees, and joining the Durham Coast line north of Billingham, with a north-south platform serving Middlesbrough? Heading due north from Middlesbrough station, it's only 3.5 miles to the Durham Coast line, but by rail it's 12 miles and 20+ minutes. Having a line under Middlesbrough would enable it, Hartlepool and Sunderland to be served by the same services.
Stockton and Billingham wouldn't lose out, as they currently only have 1 tph each way and that would be supplemented rather than replaced (and the same would apply for the stopping Newcastle service at Thornaby, but maybe not the semi-fast 1tp2h). Eaglescliffe might lose its Grand Central services if they were able to go via Middlesbrough instead though, and I suspect Redcar would prefer to keep its Manchester services rather than Hartlepool and Sunderland getting them.
The question I should probably have started with, on the other hand, is why it takes a quarter of an hour to get from Billingham to Middlesbrough to begin with, and if there's any way that bit could be accelerated (electrification?).
Stockton and Billingham wouldn't lose out, as they currently only have 1 tph each way and that would be supplemented rather than replaced (and the same would apply for the stopping Newcastle service at Thornaby, but maybe not the semi-fast 1tp2h). Eaglescliffe might lose its Grand Central services if they were able to go via Middlesbrough instead though, and I suspect Redcar would prefer to keep its Manchester services rather than Hartlepool and Sunderland getting them.
The question I should probably have started with, on the other hand, is why it takes a quarter of an hour to get from Billingham to Middlesbrough to begin with, and if there's any way that bit could be accelerated (electrification?).