For some reason, I think Grand Central could run local services from Middlesbrough to feed on to the network at either Eaglescliffe or Hartlepool - although Hartlepool has a turning point (the bay), and this train would call at:
To Eaglescliffe:
Thornaby, Eaglescliffe
To Hartlepool:
Thornaby, Stockton, Billingham, Seaton Carew & Hartlepool.
This train could arrive at the destination around 20 minutes before the London/Sunderland service arrives, allowing on-station businesses to flourish. The local train would take 45 Minutes (on average) from Middlesbrough to Hartlepool, and so a train could run every two hours with a 15-minute wait each time allowing ample time for staff changes. This service would be a feeder for the main service, but also a service to help deal with overcrowding issues.
The service would perhaps be ran by one of the 180's that would not be in service. It definately has the popularity to do so, as the coast line is VERY busy between Middlesbrough & Hartlepool. Then Hartlepool - Sunderland & Newcastle, which is ran by NORTHERN - therefore not chipping in to profits, and will not call at any main GC Stations except Hartlepool or Eaglescliffe.