Northern Light
Superstar
The Bowmanville extension will be something like 5 peak trains a day. Thats less than whats already allowed on the Milton Line. So the precedent has already been set.
What CP doesnt want is some express, electrified all day service in their corridor, which is what the discussion is about.
I believe the HFR plan will find CP to be hard to deal with, and will do something like route the VIA trains down the Stouffville Line with a connection to the Peterborough sub.
The Peterborough sub is expected to be outright bought by VIA, so that is a similar solution to the missing link in Toronto with Metrolinx.
Bomanville is pegged for 2-way, All-day, last I heard.
Though likely at a much lower level that the core of LSE, maybe hourly.