I believe I have the best solution for the Upfield line.
The line should be tunnelled, with 7 storey high apartment complexes built above it.
Park St to Bell St should be tunnelled, it is approx 4150m.
I will presume 3200m length above the tunnel is for apartment construction.
3200m length x 13.6m width x 6 storeys = 261,120m2.
(1 level is dedicated to car parking.)
An very quick look at realestate.com.au, I'm guessing average 3 bedroom apartment is 100m2 and $530,000.
So total sales roughly will be: $1.38 billion. ($1,383,936,000)
i am making a total guess, but I will assume construction cost is 60% of the sale price, $828 million.
It appears the cost of twin 15km tunnels is $2.8 billion:
I am only guessing, but based on Sydney Metro, it should cost about $1 billion to build 4km cut and cover tunnel plus new train stations.
Then cut and cover tunnel should be 3 tracks wide - make sure there will be capacity for possible future of Metro express / Vline passenger / freight.
My rough back of the envelope maths, shows Victorian Government can tunnel the Upfield line for a net cost of $1,380 milllion - $828 million - $1000 million = $448 million.
A net outlay of $450 million for a 4km rail tunnel is a bargain. Maybe with more accurate estimate, it could be even less.