It depends on how serious the government is about making VLine SG (we know that Metro never will).As long as the $15B includes Airport-Wallan and Airport-Sunbury/Gisborne (depends on how straight you make the track) for VLine services as well as platforms 17/18 at southern cross I think it will be money well spent. There should be enough space for above ground until Footscray station. No reason it can't dive into a tunnel after the marribynong.Ah yes, but Sunbury/Gisborne > Bendigo depends on the gauge doesn't it!
SG MARL can take SG'd Seymour / Shepp line trains
BG MARL can take 'em all - Bendigo BG and BG Seymour/Shepp
If MARL is BG, trains can be throughrouted to Bendigo and beyond, as well as Seymour (And shepparton until SG conversion).
Unless the Government commits to SG the whole Bendigo corridor, or the new "Geelong Express" service, I think MARL will be BG.
Wurundjeri Way currently has a massive median strip that could be cut down.
1. Southern Cross/Spencer St's platforms are reconfigured for the eventual MM2
2. Loop rationalisation program
3. RRL to Gippsland
4. Removal of turnback roads as more and more services become DMU+EMU
Then I think there is ample space for additional platforms between Spencer St and Wurundjuri way, even if some end up underground for MM2.