I don't think Southport itself is that significant - but what is is the Atherton line, which is as close to Merseyrail as anything TfGM has. That absolutely should be under TfGM's remit, and post Westhoughton electrification it's fairly likely at least one of the two Southports will go that way - most likely the Victoria one as Atherton services traditionally have gone to Vic. Though it is worth noting that TfGM's area does go all the way to Appley Bridge. Continuing it to Stalyvegas may be more debatable but could still happen even as a DMU if Vic is getting a bit crowded.
Truncating Southport to Wigan is not going to happen regardless of Westhoughton wires. It's too valuable a through service.
OTOH Kirkby possibly isn't mentioned because there's a fair chance it will be Merseyrail by then?
TfGM mentioning Southport is significant, as is Southport - Manchester
Piccadilly connectivity and the fact that Greater Manchester destinations Gathurst and Appley Bridge are on the Southport line. Atherton doesn’t work since all that has survived is an all stops Sprinter which by design was only ever tertiary to both expresses and freight. It needs to be 4 tracks throughout with a
very frequent all stops service but Wigan/Southport/Kirkby services running non-stop and not conflicting. The line speed also needs to be
much higher than 50mph.
Southport - Wigan shuttles are not possible, either politically or with the current track layout, but since they didn’t hesitate to scrap Southport - Piccadilly, what may go is Southport - Bolton though, with 2tph EMUs doing Wigan - Stalybridge and the Southport going via Atherton, possibly to Leeds again, rather than DMUs under the wires. If they aren’t energised until 2025, the 769s could be gone by then and they could always run Southport - Rochdale.
I am conflicted as I do not support all Southport services running to Victoria and not Piccadilly, or running DMUs under the wires, but I can’t wait to be rid of the Southport train which is already full and standing past Bolton stopping at Moses Gate, Farnworth or Kearsley.
There seems some logic in hiving off the Hadfield/Rose Hill/New Mills group as they are self-contained (but why not convert them to tram?). Don't know what you do about NMC-Chinley though.
Wigan-Hindley-Victoria(-Rochdale/Stalybridge) would also make some sense, certainly better than Golborne.
I suppose you could invent a Bolton-Piccadilly-Alderley Edge stopper, but is that the best use of Castlefield paths? Or Rochdale-Alderley Edge?
The GCR lines are not self contained with TPE to Stalybridge or New Mills Central - Sheffield. Do Cleethorpes services divert this way? You could requadruple to Guide Bridge but what about freight crossing the formation there? Reopening the L&NWR Guide Bridge avoiding line anyone?
There seems to be no good use of Castlefield at this point, but a local stopping service for the Ordsall Chord would be much more useful than the Saltburn.