61653 HTAFC
Veteran Member
But the key difference is that Merseyrail has exclusive use of most of its current infrastructure barring a few minor points. Having it interwork with other services would make it less reliable and more complex. Warrington could be done by handing over the CLC west of Warrington Central to their exclusive use (and wire the other half with a 15 minute frequency EMU service to Manchester), running the expresses another way, but the others couldn't.
Yes, extending Merseyrail onto the "city line" services would introduce complexity, nobody is suggesting that it wouldn't. But I know you have often compared Merseyrail to a German S-Bahn type service: something which also interacts with other services away from the central core. Something being difficult doesn't necessarily mean it shouldn't be done. If our ancestors had the same fear of complexity our railways would still be horse-drawn.
Anyway, the OP asks if Lime Street needs more capacity, and as it is constrained by surrounding buildings one way to create capacity would be to use the Wapping Tunnel and divert inner-suburban services onto Merseyrail.
I'd avoid the Warrington Central idea though, in favour of Bank Quay via Huyton. Complete 4-tracking should be possible once that motorway bridge at Broad Green is due for replacement (due fairly soon AIUI).