I was thinking more of just the odd train a day. If you take the 2D84 08:45 from Whitby for example, this arrives in Middlesbrough at 10:15. The 1P68 TPE leaves just 6 minutes later going to Manny Airport, so the path already exists for this Northern service to be extended to York. I am guessing it would not be able to keep to the same timings South of Northallerton though unless you could put a 170 or 195 on it. Interestingly, the 1B25 York – Blackpool leaves York only 6 minutes after 1P68 arrives, so this opens up the possibility of extending the train even further to Leeds and beyond. That really would open up the connections..and no change of train either.