The situation at Leeds has a fairly obvious problem-one of the throughs (platform 8) is needed to stable EC stock for London runs throughout the day, and so can't be used. 15 & 16 are needed for the regular TPE services, and there's been talk of a 5th and even a 6th hourly train through them, the delay potential with services from far afield is huge. This leaves 9, 11a, 11c and 12c for through services, as 12a is, like 10 and 13, used for stoppers on the Wakefield/Caldervale lines.
The first thing that can be done is to isolate the northeast side of the station-that is, trains heading out via Armley (i.e. Airedale/Wharfedale/Harrogate lines) to not cross any other lines. By constructing 2 new platforms to the north of the current platform 1, and reinstating the most northerly approach line, you can isolate this entire section. If running 6tph to Apperley Bridge Junction, and 4tph to Harrogate, there should still be enough space, if what will then be platforms 1-6 (current 1-4 + 2 news) are used.
Of the existing bays this leaves 6, again, used for EC services, and 5. 5 is currently mainly used for services over the S&C & Little North Western lines. Personally I'd like to see an hourly frequency on both of these, and potentially an hourly Glasgow service via the S&C and GSWR. Even so, 3tph should be easily coped with by 5. So that's that sorted.
This is then where things get tricky. For we now have 9, 10, 11a, 11c, 12a, 12c
and 13 to run both Wakefield line & Caldervale line services off. If the assumption is that 9 & 11c are XC platforms for through services, 11a, 12 & 13 to the rest. Giving 10 & 11a over to EMUs on the Wakefield reduces cross-platform clash, even if you're running 2tph Doncaster 2tph Sheffield. 6tph on the Caldervale will be tight on just 12 and 13, so again, problems here.
Worst is the south end of the station, where if you wanted to run, say, 2tph Knottingley, 2tph Sheffield and 2tph Sheffield (express) via the Hallam/Pontefract lines you currentlh have solely platform 17. A new platform 18 (which would, of course, with our two new platforms already become platform 20), connected by angled bridge & travelator from the current concourse to the old Coal Dock could relieve some of the pressure, but still, it's a tight fit.
So even for the western approach, if you increase service to the levels desired by some people (certainly the Five Towns are keen for more services), then it's really going to have to be four new platforms to the north, in order to take the wakefield line services as well as the A&W/Harrogate ones. This basically obliterates Leeds station carpark.
Moreover, still there's a problem in the east. With 9 fast services, and still no York/Selby stoppers accounted for (2 each would bring the line up to 13), there's a bit of a problem getting 13tph along that viaduct. By building a station at Marsh Lane, you can firstly take the stoppers out of the eastern bays, reducing the number of trains you have to force through the viaduct. You also create a much smaller isolated section, whereby as soon as one of the Marsh Lane through platforms is freed, the next train can be cleared to depart City.
Also, as a Leodensian, a station at Marsh Lane serving the newly regenerating area at Quarry Hill, Clarence Dock & inner city east Leeds would be very beneficial to the city.
