Eddington wants to spend $10B on a 20km tunnel across melbourne. Even assuming people would want to use it, here is a cheaper route which, as far as I can tell, provides the same service:
Basically a tunnel under the yarra at the mouth (would have to drop about 30m, which at a slope of 2% requires a lead of 1.5km suggesting a tunnel length of about 3km. Williamstown road is almost unused and the line can run along side the light rail on the old Port Melbourne line (reverting the bridge to rail use, or spending some more money for a tunnel or new bridge).
And with all the money saved, the government can do all the other things that need to be done, like East Cranbourne, South Morang, Outer circle, and Rowville extension past monash. double the number of trains run, and fix the signalling. It's amazing what $10B will do if you spend it wisely.