With all due respect, I think you are comparing apples with oranges a bit here, with pre and post signal upgrades - though I'm not convinced I have not mis-understood you.East of Strathfield, the Main has 12tph (4xT9 + 4xCCN + 4xBM), all running to Sydney Terminal.
14tph on the Mains from the North leaves 6 unused slots, and that is the extra capacity that could be unlocked by a sextup ...
The Suburban has 20tph (16xT1 + 4xT9) all running through the CBD across the Harbour Bridge.
The Main and Suburban with the existing signalling have a potential combined line frequency of 40tph.
With the existing combined frequency of 32tph, that leaves 8 spare paths.
Without the T1 sextup, they can't be used for increased T1 or BM services, but some could be for T9 and CCN.
Check, I follow you so far.
The signalling upgrade will increase the combined line frequency for the Main and Suburban to 48tph, an increase of another 8 paths.
That's a total of 16 extra paths with the signalling upgrade, ...
This is where you lose me. Granville - Strathfield will also be upgraded to 24 tph, taking your upgrade back to 12 extra slots (half a track pair).
As a side note, I could argue the suburbans between Granville and Strathfield also get another 4 slots (and have 4 spare), if you are prepared to "mix the streams". Conceivably the Richmond Line could be taken off T1 and added to T2 (or given a new number) to use the extra slots ATP offers, freeing up more on the main T1 route: Penrith-Sydney.
But I think post NIFS, there will be another 4 Central Coast train not going via Chatswood, bringing the Northern line up to 16tph, leaving only 4 slots free, and 8 post ATP ie 1/5th of a track pair, then 1/3rd of a track pair released by the sextup.
That's why I think that sextup offers little value. The likely possible utilisation of the 3 track pairs Granville/Lidcome to Strathfield with sextup will be only 2 track pairs.
p, which could be utilised if further track amplification by way of an express tunnel were to be contemplated between Strathfield and the CBD.
I'm with you 100% here. If they built that, then definitely the sextup is worthwhile.
The other factor to be considered is that the sextup will allow the T2 Parramatta services to be shifted to T1, freeing up extra paths on T2 and completely separating it. I'd consider the shifted T2 services to be additional services for T1 and it also opens up the opportunity to extend them as an all stations service to/from Blacktown.
I agree that is an option - and potentially a good option. That would be an extensions of the current T2 ex-Parra services, and perhaps another 4tph pattern when ATP releases another 4 slots on T2/Inner West. But I don't see why the sextup is necessary for that (at least not for capacity reasons).