I do wonder about that “relatively short distance” bit. You’re definitely right about the large number of commuters, but considering 60% of it is going to be tunnels, and a further 20% via bridges and viaducts, that seems likely to be much more expensive per kilometre than some other routes might be. Someone else suggested Sydney to Canberra, which would have added prestige factor and be connecting different states/territories, demonstrating more clearly the need for federal involvement. I imagine it would have a much lower per-kilometre cost, too. Starting with Sydney to Wollongong (on the way to Canberra) might achieve a fairly similar level of demand and time savings to Newy.