I'm not aware of previous iterations of this proposal affecting 50km of track.
Generally 2WAD was proposed to run to somewhere between Erindale to Lisgar.
I was privy to some of the previous estimates, and I can't wrap my head around 6B, construction inflation or not. Have to see what's in this estimate. The suggestion above by
@rbt that it might involve corridor acquisition from CP (ownership as opposed to running rights) may be a partial explainer, TBD.
@ARG1 may also be on to something.
Whatever the case, 'simple' filling out of missing sections of third/fourth track doesn't add up to 6B for me.