A lot of info there, thanks.
It has been suggested a few posts up that another 4 ish departures an hour will run on the ECML at some point. Which suggests there are paths available.
Another alternative could be to extend the existing Great Northern service from Peterborough to Doncaster. And use that path south of Peterborough. And perhaps reduce some stops on that if it was needed to. Although either way connectivity would be greatly improved even with the same stops. Especially north of Peterborough.
And as I suggested earlier, there's already a class 90 set which runs at 110mph so why would 125mph be a necessity? A 110mph EMU should be adequate even if it were to run on the fast line.
It wouldn’t be feasible to use a GN path at peak times - many of the fast Peterborough services are already maximum length and leave with most seats taken. One suspects these services will increase in popularity over time as they will offer a more comfortable journey experience compared to class 700s.
Off-peak in theory both Peterborough GN paths are now handed to Thameslink, and call at too many stations to provide a useful London service from places further north - unless you’re thinking of there being some competition on price which I don’t see being desirable on a route with constrained capacity.
I’m not sure I see the merit in extending GN beyond Peterborough. The current VTEC London to Newark and York services effectively fulfil the same function, but offering Intercity stock instead of commuter. It’s already easy to feed into these at Stevenage or Peterborough. There would be some benefit in extending semi-fast GN services beyond Peterborough simply to give more through opportunities perhaps from places like Welwyn or Hitchin, but you’re still only offering Grantham, Newark, Retford and Doncaster, and at the cost of 100 or 110mph trains. You also have the issue of where the GN trains would be maintained or at least stabled at the country end.
Sure, journeys like Hitchin to Lincoln or Hatfield to Grimsby would become possible with one change instead of two, but are we really talking about a massive market?
The WAGN proposal cropped up at a time when the ball game was rather different. Not so many trains overall on the route, a structure which was more encouraging of competition, some brand new high-quality commuter trains many of which stood idle during the daytime, and perhaps a bit of a gap in the market. I think things have moved on and we now have basically the same thing but with VTEC instead. Find a way of stopping one IEP an hour at St Neots and Huntingdon and you basically have the same thing, but arguably better!