>>1343454Ah, I see. There were 3 main alignments for the airport line - branching from the Craigieburn line at Broadmeadows, via Sunshine, or direct tunnel via Highpoint from either Footscray or the Flemington Racecourse line. Direct tunnel would be too expensive and there is all the contaminated soil from the former Defence base at Maribyrnong to deal with. The Federal government preferred this route, however, as it would allow them to redevelop that federal Defence land into a new housing estate, contamination be damned.
Additionally there's too much congestion along the Broadmeadows route, which would squeeze Craigieburn, V/Line and Airport trains onto a single track pair. Realistically you couldn't add much more capacity unless you diverted V/Line along the Upfield line, which would introduce more problems such as rejoining Upfield-Roxburgh Park, the large single track section past Gowrie or the numerous level crossings. They can't really branch from Essendon anyway because it would largely parallel the 59 tram for little gain, and upon rejoining the main line you run into the aforementioned capacity issues again. It would also have to be in tunnel and owing to the built up areas around the tracks there isn't really space for a tunnel entrance.
So, the Sunshine route is the most viable because about 3/4 of the route already exists in some capacity, following the existing lines to Sunshine then along the Albion-Jacana freight line. Sunshine also provides the best connectivity to the west because of the planned Sunshine hub.
