The capacity issue between Marsden and Huddersfield is similar to that between Ravensthorpe and Leeds after completion of the four-tracking from Huddersfield. The 8TPH of fasts doesn't have to be regular, it can include a couple of longer gaps for a stopper in between. Assuming electrification via Stalybridge that stopper will be an EMU, so quicker off the mark than it is today. There might also be some semi-fasts via Stalybridge, but these would be non-stop on the shared section so would be able to keep up with the NPR trains.
I wonder if the third track is more about freight.
It's decidedly unclear what the arrangements at Marsden will be.
Will NPR reuse the old bores of the Standedge tunnel or bore a new one on a possibly different alignment? One 2-track bore or two single track bores?
How will the TRU and NPR routes converge at Marsden, ie grade separated or not?
I think it's a fair bet that legacy TRU trains will have a slower run through the area, on top of the 45mph on the curves at the eastern end of the tunnel.
2-up, 1-down tracks, all bi-di with ETCS, would make sense for a 3-track Marsden-Huddersfield section, but the new 3rd track will have to wander about a bit.
The existing 2-track route changes sides on the old formation, and in places there were essentially two separate 2-track routes.
It will be an interesting design.