To be honest overall I find it somewhat surprising that this stretch of track (Strathfield/Hornsby, or at the very least Strathfield/Epping) has yet to get the full quad given the numerous freight, regional, intercity and suburban services. I could totally see a limited stops suburban stopping at something like Epping, Eastwood, West Ryde, Rhodes, Strathfield, Burwood, Redfern, Central. Rhodes has had huge development and has a shopping center. Epping continues to grow in importance as an interchange. I saw a comment someone had made expressing concern about the amount of space left between the dive and main up at North Strathfield to re-instate the relief/outer up so that regional/intercity trains can use it to get to Strathfield un-interrupted by suburbans which can use the existing main up. Below are a few photos courtesy of highplans68 on flickr. I'm guessing they'd have to turn the Pomeroy Street Bridge to a single span if possible? because one of the supports would no doubt block the re-in statement of the up relief.
The stretch of track between Strathfield and Epping will get the full quad, as well as triplication from Epping to Hornsby, with completion of Stage 2 of the Northern Sydney Freight Corridor program. It's programmed to be completed within the next decade.
Limited stop peak hour services from Hornsby to Sydney Terminal will be implemented following the shutdown of the Epping to Chatswood Rail Link for metro conversion in a couple of weeks, running all stops to Eastwood, then West Ryde, Meadowbank, Rhodes, Strathfield and Central. In the off-peak, it will be all stations to Burwood, Redfern, Central and through the CBD to the North Shore.
Space has been reserved for reinstatement of the Up Suburban/Relief Line through North Strathfield to Strathfield, although constructing an additional platform is unlikely.