This blog in October 2015 proposed linking up the upper North Shore line stations (north of Killara) to Sydney Metro Northwest (which has tunnels running underneath Lindfield just 2km from Killara and needed to be shutdown in 2018-2019 anyway for Metro refurbishment). This would have enabled Hornsby to CBD services in 25 minutes as well as providing additional capacity for the upper North Shore line (being the equivalent of quadruplicating tracks between Chatswood and Gordon by making better use of unused tunnels).
In March 2018, the TfNSW Western Sydney Rail Scoping Study provided TfNSW projections for 2056 rail capacity utilisation. These projections show the upper North Shore line will have "Overcrowding limiting reliability" (RED line north of Chatswood in map below), whereas Sydney Metro Northwest capacity will be 60% unused (GREEN loop near Chatswood).
The opportunity cost is not just that the $10 billion Sydney Metro Northwest capacity is 60% unused but also that higher operating cost double deck trains need to be used on the upper North Shore line instead of lower operating cost single deck trains. Additionally Sydney Metro loses out on having a potential turnback site at Gordon and has to send empty carriages all the way to Tallawong (increasing operating cost).
