(02 Jan 2014, 11:17 pm)Tom wrote What's the current issues like?
Until they changed the Peterlee services into the current guise (back when the 22 avoid Peterlee, and the Peterlee-Hartlepool services were 23/24a/24b) the 23 was interworked with the 22 to enable driver changeovers to take place in Durham on the 22. Now, as the 2 are combined to give a 15 minutely Peterlee-Hartlepool, this doesn't happen, nor does the 23 interwork with the 24. As such, driver changeovers happen in Sunderland using staff cars from Durham.
Extending the 23 to Middlesbrough would then allow the operation to pass to Stockton and enable drivers to change there (although, depending what route they took from Hartlepool to Middlesbrough would then depend on driving hours).