This is a type of discussion I usually stay away from. I like dealing in verifiable facts, but here's a bit of crystal ball gazing, a bit of rumour, and some facts that I will not verify the source.
Porthaul currently carry containerised wood chips from Mt Gambier to Geelong by truck. They have purchased a large block of land on Darts Rd Portland adjacent to the rail. The rumour is they want to create an intermodal siding so they can transfer containers to rail.
There's also a mill that's just opened in Portland for the sole purpose of cutting square sections between 90mm x 90mm and 150mm x 150mm in lengths up to four metres long for export in containers. The material would be sawn down to smaller sized boards in China, then used to manufacture timber packaging such as pallets. Shipping via rail to Geelong is likely if a siding goes ahead. Chance of siding within 5 years happening 70%.
Most hardwood logs and chips come from plantations north of Heywood and around Casterton. Chances of movement by rail 0% ever
Softwood is mostly harvested south of the Princes Hwy. For transport, the entire line to at least Rennick would have to be reconstructed, and sidings built. Chance of happening - very optimistic 10% in 20 years
Mineral sands. Balranald may fire up within two years with sand going to Hopetoun for transport by rail to Hamilton, then export via Portland. The company has also been testing new refining techniques at Hamilton to allow them to process the spoil at Ouyen that couldn't be initially treated. If that comes off, more shipments to Hamilton / Portland. This is likely to make the Geraldton plant redundant, so all raw material will be shipped to Portland for transport to Hamilton for treatment. Rail opportunity within 5 years 80%.
Grain. Sporadic. I think the Port would love to bulldoze the silos to provide more room for their year round money maker (timber), but the chance of that happening - maybe 20% over 10 years. Chance of increase in area - 0%.
Major (or even minor) container port development. Never going to happen. Too many bulk materials going out and no land for storage.
Passenger rail. Currently it takes about 5 1/2 hours by bus and rail to Melbourne via Warrnambool. Going by road via Hamilton and Ararat is about 400 km and takes 4 1/2 hrs. Probably won't be much time saving using rail as it would probably be restricted to 80 km/h between Portland and Ararat. My guess is nothing will happen unless the rail is upgraded. Ten year outlook 50/50.