Im no expert, but I'm quite sure that areas like North Sydney, St Leonards and Chatswood that are actually along the rail line are much more densly populated than the hills. Its clear from the amount of apartments in these areas. North West link is not viable. Too expesive to build. North Shore Line works because the infrastructure was in place when the area developed. There is simply no room to build the North West link without the expenses of tunnels and land reposession which I mentioned earlier.