There were an number of options for this: extending the dc (now battery) units via the south curve to B Bridge and bringing in the Preston DMU via the north curve. The case against is its addition of an expensive complex junction to a minor, subsidised line (look at the entries and exits for Rufford and Croston) and the limitations of the existing two platform station needing to be kept clear for the Southport services. An alternative was a new interchange where the lines physically cross, in the middle of a field a few hundred yards from the bridge. Just three platforms and two ramps with no expensive track and signalling; with a car park, you could have a "Liverpool North Parkway"!
Whatever comes or doesn't of this, it is a no-brainer to use the 777's on the Preston service; 18 miles is a convenient run on batteries. That would improve use!
I remember seeing dining cars (amongst other regional services) in Exchange Station, serving this line directly to Scotland - what a come down, a planned decline for Lancashire.
WAO