Im sure an extra 27mins by train from the cbd of melb to VFL park wouldn't have been an issue?
Express from the city...maybe 27mins, probably a fair bit more...27mins would be unlikely considering the need to run other trains on the Dandenong/Pakenham line to service the other stops (and those trains would get in the way). More likely they'd run 'heavier than usual' services to Huntingdale and an express shuttle on the special line from there to VFL park.
Also, unless they forked out for 2 lines from Huntingdale to VFL park (which would be a very tight fit on the median strip up North/Wellington Rd). You'd only have one train running on that strip...which would probably be a 10-15 mins return...Let's say you could double up at the end, and it's set up to get green lights at all the major intersections, so 1 every 5 mins.
1000 people per train, so 12,000/people per hour....It would take a while to fill a 155,000 stadium that way. (The MCG has Richmond station...10 platforms + Jolimont with 2 more, and a lot of people still walk from Flinders St, Parliament or wherever)
In short, the train line would help a bit, but not that much.