When the Inland opens SG trains will have instant access to the port on a section of track that is barely used by SG services.
The 9km section DG from Salisbury to Dutton Park is shared with sparks and not suited to DS, to do so would require
- Building a dedicated 4th track,
- Rail over pass of the junction to Corinda (same as they did in Adelaide)
- Removal of the unused OH from Salisbury into AR yard.
- Potentially lowering the track bed under the odd bridge (I haven't read the report yet)
None of this is exceptionally hard and the corridor is wide enough for most of the route except the last bit close to Dutton Park fly over.
However to add some complexity to the situation it won't be a 4th track, rather it would be a 5th track as the Qld govt one day in the distant future will build the Greenbank / Beaudesert line which parallels the DG and interstate and completion of the CRR project. Again its one of those things the Qld govt and Feds should get together and plot out a jointly funded solution (with the caveat Qld has a notorious history of being dependent on Feds for suburban rail upgrades).
North of Dutton Park, there is no O/H all the way to the port and not shared with sparks and should not be an issue with the exception of the odd older bridge (maybe), an easy to fix proble, by sinking the rail or building a new bridge. Nothing that hasn't been done before.
Brisbane Port is currently open to 900m SG trains, some changes will be required for longer or shunt at AR. I would however say demand much drive this spending, not guess work.
Is any of this required now, no, is any of this required from day one of the Inland opening, mmm, not sure but the answer would likely be no, but follow the Inland project opening again part of a major upgrade of the Sals - Dutton Park corridor for the Greenbank line on what would be the worst designed piece of rail infrastructure in the country.