Ok, firstly its good to see a bit of convergence on thinking about this issue. The tunnel should come in at under $2B if the NWRL is anything to go by. And it would save money to have minimal stations. I've chosen Olympic Park and Five Dock. Your mileage may vary.Yes, that seems to be the case. It didn't appear like that in Sydney's Rail Future as we were discussing but it seems that is what will happen. Its needed to facilitate the growth on the stations from Strathfield back to Parramatta anyway, post WestConnex its forecast that these stations would see lots of development. I think they are ripe for it too.
This relates to your comment and the comment of Eco-Rat, I have to say I thought that the better mid to long term option is not what they have chosen for the Western corridor. The $1B Western Sydney Rail Upgrade Program makes sense on a few levels but I cant help but feel if they had of gone with an option such as leaked report that recommended the Parramatta to Croydon express tunnel the cost would not be that much more and the result much better.
Although even to Croydon does seem half done. Extend it to Sydney Terminal. Such a line could be built with no stations to keep the cost well down. Western line trains run through the existing Harris Park station station dive into a tunnel before Granville (somewhere around Wigram st perhaps) then runs directly to the CBD to surface somewhere near Regent St Morturry and into Sydney Terminal. This new tunnel can then take all 20 x 10-12 car TPH from the Main Western line to transfer to metro into the CBD and the North Shore.
Surely around $2B would not be out of the question for this tunnel? It would have good political capital too, an all new piece of infrastructure and assuming 160 KM/h capable trains an 11 min CBD journey would make it a much easier sell with the forced transfer to Metro.
You can then leave the existing main lines as they are with 10 TPH 8 car trains on the Western locals from St Marys or Blacktown all stations to Strathfield joining the 10 TPH from the Northern line up to the North Shore. You could in addition run 6 TPH 10 - 12 Car Lidcombe > Olympic Park > Sydney Terminal trains too via the Mains (joining the Central Coast and Newcastle line trains). Then you have the 12 TPH Main South trains run express from Merrylands to Homebush on the locals to join the 8 TPH inner west line and then all stations to the City Circle. Cumberland line trains are bumped up to 6 TPH.
If you really wanted to make the Liverpool/ Main South journey quicker another variation on the above with the new Western tunnel would be to build to build a flyover at Granville Junction to allow Liverpool trains to join the Mains and run express to Strathfield and join the Northern line trains on the Suburbans to the North Shore. Again probably 10 TPH each for the Main South and Main Northern lines. You could still also tun the Olympic Park express trains mentioned above. Then run 12 TPH Blacktown starters on the locals all stops to Homebush to meet the 8 TPH Inner west line to the City Circle.
When you have soaked up the capacity on the Metro for large scale transfers (remembering the Main Western line is Sydney's busiest) you then consider something like the CBD Relief line to Wynyard etc but I honestly think that first up the fast tunnel is the most cost effective. least intrusive and easy to implement option plus it is its own sector so can be privately run.
Secondly, I still think it makes life more difficult west of Parramatta to integrate such a tunnel with the existing tracks there. I know we've argued this before. I'm hoping we'll continue to converge.
Thirdly, if you can get the "south" part of the inner west and south line completely off the map then that simplifies life a whole lot. Its not that complex, you just create a Y junction so that you can run Liverpool trains to Central via East Hills. Now you don't have to worry about complex stopping patterns and the entire inner west line can run as a sweeper service at high frequency.
Fourth, if you can run 12 car trains from the west, forget about Penrith and the Blue Mountains for a moment. Just upgrade the platforms at St Marys, Blacktown, Parramatta (maybe Strathfield) and then Central. Then you get a big capacity upgrade on the express tracks for very little.
Fifth (I know this will run counter intuitive to some) the inner west line simply runs to Liverpool. No Granville to Parramatta service. No more flat junction conflict and you can run Cumberland trains at higher frequency. With a high enough frequency, Granville to Parramatta via Merrylands adds a few minutes.
In short, you have two track pairs from Parramatta to Homebush. The express track pair carries your 12 car trains, some T1 trains (which don't go west of Blacktown) and of course the Blue Mountains trains. With 10x12 car trains in the mix, that track pair now carries the equivalent of 25x8 car trains. The local track pair exclusively carries the inner west sweeper service to Liverpool.
Now, if you were clever, you could interleave the inner west trains with the Cumberland line trains one to one and the Cumberland line trains could run limited stops from Liverpool to Parramatta.