I've just read about this in the paper - reducing the number of platforms to three sounds like two through platforms (like Deansgate) and one terminus - the current P5. If that's the case I'm struggling to see how you would get more trains through and/or quicker, at least with 4 lines, a stopping train can be overtaken by a fast one for example?
Overtaking stopping trains is more theory than practice today.
A few years ago I went on a morning down service which used platform 1 and was overtaken as booked by a train using platform 2; my train went on to call at Deansgate which was my destination. But platform 1 is avoided because it’s narrow and has no step-free access.
In the up direction, overlaps restrict the possible overtakes, and although it’s possible there are restrictions such as it’s only possible to overtake a 6-car train with another 6-car train if the overtaking train uses platform 4 when the overtaken train is in platform 3, the other way round is not possible.
The only real use of “overtaking” today is during disruption when trains are held up for whatever reason. It’s this flexibility which most of us have commented on, the new station will allow more trains in normal operation.
EDIT Attached from the 2017 timetable, which shows 2N55 being overtaken at Oxford Road by 1F92 (I used 2N55 myself); since the 2018 timetable pretty much nothing is now "booked" to use platform 1 apart from freight trains I don't think.
