It is precisely for the reason that the least work would be required to connect the Bidston - Wrexham line to Merseyrail that this should NOT be attempted in the first instance.I agree, it's most likely the Wrexham line will be next. It has a good business case as it requires little work to get some form of service running.
It is currently possible to get from Bidston to Wrexham by rail, as well as Liverpool and West Kirby. The line will shortly enjoy a doubling in frequency to half hourly and longer trains in the form of the 230s. These should be allowed to run the route for at least 10 years, until after 150s are withdrawn, before being replaced by additional pure battery 777s.
By contrast it is not possible to get to Southport by rail from Ormskirk or Preston, or to Skelmersdale from anywhere, so the priorities for network expansion should be where work is required, but will create the most new travel opportunities.
Any extension beyond Ormskirk or Kirkby will require the partitions to be removed and an additional few feet of track to be laid in the middle. Both could also do with an additional platform for increased resiliency, so the work is not trivial. Resignalling is unavoidable for most extensions. There is no signalling of any kind north of Ormskirk, or between Southport and Burscough Bridge.
Relaying track over the unobstructed Network Rail owned trackbed of the Burscough Curves and properly integrating the Ormskirk - Preston line with the line to Southport via both reinstated Burscough Curves, as well as the WCML and existing Liverpool - Ormskirk line and increasing capacity between Southport and Burscough Bridge by a factor of 8 by installing additional signals would serve as a good test bed for the later construction of an entirely new embankment from between Rainford and Upholland to Skelmersdale, as well as the other resignalling projects and redoubling needed there and elsewhere.
On the Wirral Line, the first battery equipped expansion that should be considered is probably Ellesmere Port - Helsby, given that effectively no work would be required and it would create a proper interchange at Helsby with the TfW services to Liverpool, Warrington and Manchester. If 777s take over Bidston - Wrexham immediately, what happens to the 230s, the Burscough Curves and the Skelmersdale branch?
