A second tunnel track isn't needed for hourly all-day 2-way (it would definitely be needed for 15-min RER, though).
Currently, the single-track section is only 1 kilometer (trench and tunnel) but can be shortened to 500 meters (tunnel only).
Only a few freight trains a day. It can be time-shared with freight trains, as it is an extremely short section, relatively speaking.
Interruptions by freight trains could theoretically be made relatively rare via a scheduling agreement, as well as multiple switching opportunities before or after a long freight train (between the junction and tunnel entrance). Given sufficient reimbursement to CP (for costs of occasionally interrupting/slowing down the odd early/late freight train) -- they'd do it -- but Metrolinx is currently sensitive of overpaying CN/CP (
overcharge controversy).
To add even better operational flexibility, one may still need to add extra track from the junction area on the east side/north side (Hamilton Junction / Bayview Junction) to near the tunnel entrance. The existing trench, all the way to the tunnel entrance, has room for 3 tracks, so an extra track can be added. It is two tracks to Locke, and one track to Queen. Afterwards, it could become three tracks to Locke, and two tracks to just before Queen. One bridge (Pearl street) will need to be re-pillared to make way for an additional track that probably would switch onto the solo track just before Queen. If this trackage is needed, I would guess that this is a project announced during the 2018-2022 election term, once they're finished with the extra track from the junction to West Harbour GO.