<Checks that this thread is in Speculative....>
So far the main objection to 'Skem Parkway' ideas have been that extending Merseyrail twice an hour from Kirkby/Headbolt Lane to Wigan would lose the through service to Manchester.
But to pick up one of the ideas from the Skem branch proposal, an idea was to have two adjacent single track lines, one of which would link to Wigan (and so Manchester) while the other would go to Headbolt Lane (and so Liverpool). This would minimise the degree of interaction between Merseyrail and the rest of the railway (what with independence from other services importing delays being one of Merseyrail's strong points)
Given that (as I understand it) the line from Rainford (and maybe Headbolt Lane - I'm not too sure of the number of tracks from there) to Wigan is twin track, could the 'adjacent single track' scheme be adapted? This would give a single track Merseyrail line from Headbolt Lane to Wigan Wallgate: I see that Wallgate to Kirkby is timetabled for 25 minutes each way, so a single track could support one train an hour from Kirkby to Wallgate and back, giving an hourly Wallgate - Skem Parkway - Headbolt Lane - Liverpool service.
The other track could support a Headbolt Lane - Skem Parkway - Wallgate - Manchester (and vice versa) service - and could the freights be flighted in behind the passenger service?
This could give two trains per hour each way between Headbolt Lane and Wigan, including one through working per hour to Manchester and another through to Liverpool. Direct connectivity is maintained to Manchester and gained to Liverpool - and there are extra journey opportunities by changing at Wallgate or Headbolt Lane.
There's got to be a catch to this, so what have I missed here?