I don't think anybody has mused about double tracking. ONR's 'updated initial business case' only speaks to adding a passing track on the Bala sub:
"A capacity study determined that rail corridor upgrades would enhance travel time and reliability of the Northeast Passenger Rail service. The study proposed a new siding north of Zephyr to facilitate train meets between Ontario Northland and CN trains. Preliminary cost estimates for this siding are included in the capital requirements of this business case, though future delivery will be determined though negotiations with CN." (page 75)
I don't know if GO has any plans for the Bala sub to serve their needs, but they only go to Bloomington anyway.
AFAIK there has been nothing outside of that passing track that I have seen as well.
If GO wants to get north of Barrie on existing track, using the Bala Sub would be the way to do it without laying new tracks. If this ever happens, this would be at least 10-20 years out before they might look at it. Extending GO on it gets them Georgina and Beaverton which are commuting towns. There are go buses, so when they see enough traffic on their buses, they might look that way.
And then there is the if... if the Northlander is a success, and the province wants to add more service to Northern ON from Toronto to places like Sudbury/SSM I doubt the CP MacTier sub would be used.It would most likely be the Bala Sub to where they directionally run like the Canadian.
And then there is the "What is to happen to the Budd RDCs" question. Maybe it gets extended south and west, or just south. That would add 3 more trains a week at current service levels.
All of this is a "wait and see", but wonder. I feel most of this is based on 3 things:
1) the success of the Northlander.
2) what happens to the HEP fleet in a few years once the Siemens fleet has arrived.
3) if GO wishes to expand along this line further. This is the least likely factor.