PS. If there are two Manchester - Liverpool slows per hour, and they are half an hour apart, must the non-stop service overtake en route (at Huyton)? What if the slows originate at Stalybridge? What are the implications of this for the speed/reliability of the services involved or is there more infrastructural capacity I've not taken into account? It all feels a bit tight to me, unless the fasts take an extended breather at Victoria as a permanent feature or the slows miss a few stops.
With current frequencies it's not a problem, on a good day anyway.
The line carries 7tph out of Liverpool at the moment (4 stoppers, 3 limited stop).
Three trains turn off at Huyton and another at Earlestown, with two joining at Earlestown/Parkside. Three of these five trains head for Piccadilly, leaving two for Victoria.
There is quite a bit of coal traffic Liverpool-WCML via Earlestown now, and maybe Port Salford to come.
The main problem is west of St Helens Jn with stations close together (the new ones really don't help).
The new layout at Huyton will allow overtaking westbound, but an equivalent eastbound facility will have to wait till the 4th track is in.
Trains can overtake at Victoria if there is a spare platform, and there is a loop at Earlestown West.
It isn't yet clear what future increase in frequencies there will be.
Maybe an extra Chester train on the eastern half, a Liverpool-Scotland portion on the western, and another TP Leeds service. Alliance? Who knows.
Class 319s should improve capacity with their faster acceleration and higher top speed.
The future problem will be negotiating the rats' nest at Ordsall Lane after the new chord goes in.
Luckily NR has just bought a Traffic Management System (TMS) for its ROCs, so we're OK!