Focusing on Stockton undermines the argument. Stockton and Thornaby stations are roughly the same distance away from Stockton High Street, and Thornaby is better suited for most of the businesses in Stockton, which are now all on Teesdale business park.
For those who can't or don't want to walk the ten minutes down to Thornaby, there's always the Arriva X66 every 20 minutes, which has just been upgraded to MAX.
The lack of a direct train between Hartlepool and Darlington is more of an issue, because the bus service between the two towns is rubbish.