There's some misinformation in that report.
(b) The proposed Doncaster railway wasn't supposed to run in a tunnel under the Eastern Freeway; it was supposed to run on the surface, along the median strip. That's why the bridge depicted has no centre pylon.
There was a tunnel dug between Victoria Park station end of the freeway reserve, and the median strip; however it was filled back in again with earth once they started postponing (and ultimately cancelled) the Doncaster railway idea as part of the freeway construction.
There was a photo of the tunnel being dug in an old Country Roads Board / Board of Works (which ever it was) report from around the time - 99.9% sure I've seen that photo somewhere on this forum.
EDIT: it was the 1974/75 CRB Annual Report. No copy though, sorry.
There's a photo of it on a rail historian's site - image 616_04: http://pjv101.net/film_bw/pjv00707.htm