I guess there is a trade off here. Running every two hours words better with the timetable as the MKIVs can be on the journeys which run non-stop between Crewe and Shrewsbury, whilst the 197s with their better acceleration are more suited to the other hour when services do stop at intermediates.
That would require changing which hour the intermediate stations get served by the long distance services though - at the moment, broadly speaking a service that runs non stop on its way to Manchester stops at the intermediates on the way back, and vice versa.
As it stands, loco hauled services are restricted to 70mph on that section whereas DMUs can do 90mph, so it would certainly help timetabling not having the loco hauled services stop.
Terminating at Cardiff does cut Swansea and West Wales off from the Borders and North West England (I used to use it quite regularly from Crewe to Swansea), however this would only be every other hour - perhaps if it all works out right the Liverpool - Cardiff train could be extended through to Swansea and West Wales instead.
No *that* is an interesting idea. Handing the Milford Haven service onto the Liverpool service would leave the Manchester-South Wales free to operate as Manchester-Cardiff by MkIV two-hourly, Manchester-Swansea by 197 two-hourly in the intervening hours. That'd leave a gap in Cardiff-Swansea/Carmarthen provision in the alternate hours, but I could see that being something that gets picked up by an improved Swanline service, or other initiatives in the area.
(Would Liverpool to Milford with couping/decoupling at Chester be too risky for the timetable planners, I wonder?)
It seems likely that the Cardiff - Holyhead and Cardiff - Liverpool services will run in the same paths on alternate hours, so extending them through to West Wales could be an option. I could see a world where these services run all stops Chester - Cardiff, and the Manchesters run limited stops alternating between MKIVs and 1st class fitted 197s. It would certainly be simpler then what it looked at one point we were heading towards, with random Manchester - Swansea MKIV services breaking up the Takt.
That said, 2 hourly MKIVs going to Manchester would require 4 sets - meaning that combined with the 3 daily Holyhead services it would require 6 sets out of 7 in use every day, which might prove optimistic.
I'm looking forward to seeing what they come up with.