I'm not really sure I see this plan as necessary, just putting aside the obvious issues that the railway line is sceduled for re-opening and re-use in the future.
We in Full Gas Peddlers are responsible for development of mountain biking facilities in Westgate Park, despite significant problems with the local Friends group doing everything they can to prevent cycling in the area. If you really want this to go ahead, you should be helping get behind us and getting local MPs and council members to support us, and stop listening to Friends every time they get on their "cycling endangers everyone else" rants - as I can assure you they do, and have been privvy to some of the reports and proposals they've put forward.
Lorimer Street already provides access to Westgate Park via the south entrance; this then goes in to bike path, which runs through Westgate Park. Current the South and North sides of the park are cut off due to Bridge Strengthening Alliance work, however in time this access will reopen, bringing the path back to Todd Road.
The next best option for this area would be to convert the fire-trail that runs from the East side of the railway line in to a sealed path, which is a VicRoads asset. I am sure if we got enough signatures behind it, we could have a wide area cleared that allows sealing of this path to provide both service vehicle access, and a separate bike lane on the other side, between the border of land owned by the Deparment of Treasury and Parks Victoria. This could then run down parallel to the old train line, still on VicRoads property, past the facilities owned by Melbourne Water and Skilled Engineering, out to Todd Road. If done correctly, this could then be joined to the path that links Beaconsfield Parade.
This would certainly fit in with our plan for allowing active recreation in Westgate Park, which is an ongoing battle thanks to a selfish few who misrepresent the user demographic of the area.
If you want to discuss this more with me, as it's something I'd be happy to try to push for, get some discussion going here, or contact me via firstname.lastname@example.org