The fact that the Slow Line 2 (both up and down) was reinstated, was supposed to enable a decent number of extra services as stuff wasn't now battling to use the slows through the platforms. In theory, all Hertford trains could just not stop at Harringay or Hornsey, and remove all the conflicting moves. Harringay and Hornsey aren't "small" stations but the likelihood of people being agitated at being stood for 90 seconds or 3.5 minutes to Finsbury Park is almost a nonsense.
Given the closeness of stations such as Manor House and Turnpike Lane and their superior frequencies and through connections to the centre of London, you don't need those Harringay and Hornsey calls.
To get to a slightly mismatched (e.g. not quarter-hourly) timetable of 8tph via Hertford, would be very positive. Especially as it would probably allow the Stevenage service to be accelerated, maybe being even fast from Finsbury Park to Palmers Green.
You'd effectively get a sort of service out of Moorgate like this:
- xx:00 Welwyn all stations.
- xx:04 Gordon Hill semi-fast (not Harringay or Hornsey), e.g. also not Grange Park. Benefits Hertford as this will pick up a lot of people after an 8 minute gap.
- xx:09 Hertford North all stations (not Harringay or Hornsey).
- xx:13 Welwyn, fast from Finsbury Park to New Southgate, possibly not Brookmans Park and Welham Green.
- xx:17 Welwyn all stations.
- xx:21 Stevenage semi-fast, non-stop from Finsbury Park to Palmers Green and perhaps not Grange Park, Crews Hill or Bayford.
- xx:26 Hertford North all stations (not Harringay or Hornsey).
And then again with 30 minutes added.
The xx:00 off Moorgate leaves Finsbury at about xx:14, and the xx:17 off Moorgate leaves Finsbury at about xx:31. Slot the Cambridge / Letchworth / Royston 'stopper' in just in front of that xx:00 Moorgate / xx:14 Finsbury, and add in stops at Ally Pally and New Barnet during high peak, and it won't quite catch the Welwyn in front. You've now got the right mix of traffic, a few extra big station stops, and less trains running around with 2 or 3 minutes of pathing time.
You also get the benefit of redeploying 8/700s off the Welwyn semi-fast from Sevenoaks and into a path departing Finsbury straight onto the fast lines and non-stop to Welwyn North, to be the high-peak Royston / Letchworth fast-ish.