If flow between Nunthorpe and Boro' was that good surely the service would be more than 1tph?
The infrastructure between Nunthorpe and Middlesbrough isn't up to the task. It's single track all the way from Guisborough Junction (just outside Middlesbrough) to Nunthorpe. A fine piece of BR 1980s cost saving. So the maximum service that can be sustained is around one an hour each way. It's why, for example, the new early arrival into Middlesbrough from December is so early. It's the only way of pathing it!
Now if someone wanted to talk about spending money to either redouble the line between Nunthorpe and Middlesbrough or at least install a passing point somewhere down the line to enable an increase in frequency I think that might have legs. The local roads that parallel the railway route are notoriously congested (it even has it's own name, the Marton Crawl!) so an improved rail service between Nunthorpe and Middlesbrough could well be a winner (indeed I think we have at least one member who uses that route regularly has commented that even the current hourly service is quite popular).
I've seen passengers at stations between Nunthorpe and Middlesborough left behind because they couldn't get on the train towards Whitby. The capacity is needed all along the route.
I think you've misinterpreted me here a bit. That was my point. The majority of traffic boards at Middlesbrough and at the stations between there and Nunthorpe on the way to Whitby. Beyond Nunthorpe however there are far fewer, if any, passengers joining the service (outside of the school train obviously) as it heads to Whitby. The same then in reverse. Overwhelming majority get on at Whitby, a handful trickle off before Nunthorpe and then everyone gets off between Nunthorpe and Middlesbrough.