You only have to look at Leeds to see what happens if a route to LHR doesn’t perform. We would still have had Gatwick if the schedule had been anywhere suitable for a north-east based passenger. Newcastle is right on the margin for a viable point to point a service to London. If rail fares weren’t so ridiculously expensive and reliability poor, then 40,000 passengers per month would very quickly become 30,000 with all the consequences to frequency that would trigger.