Does Merseyrail not have a "Northern Line" group of services? If so, that implies that it runs on the northern side of the Mersey.
Also, the Mersey passes through the Stockport area which when I was at primary school many years ago, Stockport is east of Liverpool. Also, before the Metropolitan Counties came into being in 1974, north of the Mersey was Lancashire, and south of the Mersey was Cheshire.
I was about to say the “Lancashire” and “Cheshire” sides of the Mersey is clearer. This Mersey border continued through the Non-Metropolitan Unitary Authority of Warrington to the Metropolitan Borough of Stockport, splitting both of them between Lancashire and Cheshire, as well as some other areas.
There is no reason to split the fleet between the Northern and Wirral lines. Class 507 and 508 units travel between the two lines every day as empty stock workings and Kirkdale will be the main base for the whole new fleet. Should it eventually be decided to equip some 777 sets with batteries to permit operation beyond the current limits, then some form of segregation may be desirable, depending upon which services are affected. For example, if battery operation beyond Ormskirk to Preston came about, but this was the only line where battery operation was introduced, you would not normally want your battery equipped sets to be allocated to Wirral Line services.
If certain sets are ever fitted with batteries, 25kV equipment or other non-standard additions, this will parallel the Manchester Metrolink, who’s “standard” fleet of M5000s was built in many batches at different times, to run on a number of different lines, built or converted to Metrolink operation at different times over a now almost 30 year period, each requiring different equipment to be fitted which was standard at that time. Therefore not all trams can run on all lines.
These non-standard 777s may be designated as 777/1 or 777/2 etc to differentiate them. If battery operation is introduced on both Ormskirk - Preston and Kirkby - Skelmersdale sections as well as Ellesmere Port - Helsby, battery sets will still be interchangeable between Northern and Wirral lines. I believe all 777s will be allocated to Kirkdale, with some stabling overnight and light maintenance taking place at Birkenhead North. The TfW 230s will be allocated to and maintained at Birkenhead North once the 508s have gone.
It is called the Northern Line, but it's not a particularly well-named line.
Squirrel Line would be more appropriate.
The Northern Line services were designated as such in 1971, before the opening of the Link tunnel or the Hunts Cross electrification, when all Northern Line services ran North from Liverpool out of the original Liverpool Exchange terminus, including the Diesel through service to Wigan and Manchester via Kirkby. The Preston line had already been severed at Ormskirk.
This naming remained even after the new through service began from Kirkby to Garston, on the South side of Liverpool. When the separate Gateacre service ran out of the Liverpool Central high level terminus, it may have made more sense for this to be a separate Southern Line. The other Northern Line on the London Underground began life as the City and South London Railway, so it seems no one is able to decide successfully on a geographically accurate compass point designation.