Just to pick up on this, at present NO Northern Drivers sign Ferryhill - Norton (a.k.a "Stillington") and there are no immediate plans to do so. Yes they applied for rights to operate there but couldn't get realistic timetable slots so no plans for route learning or being timetabled in. Plan B is an extra train an hour via Durham Coast but Covid keeps putting that off. On another note even though 158s daily get swapped around via the Settle and Carlisle, quite often there is a need to get one to Yorkshire quicker for unit balancing / wheelflat turning (no wheel lathe at Heaton) so the Darlington to York route knowledge is needed
Getting back to topic the reasoning is correct for the "Z" trains and its not just isolated to this route, they've already been noted on the Hellifield "Dalesrail" line, there are route learning trains around the North East (Darlington / Middlesbrough to Newcastle) and probably a few more around to network