You should read the Report on it.
https://www.toronto.ca/legdocs/mmis/2022/ex/bgrd/backgroundfile-224481.pdf and
https://www.toronto.ca/legdocs/mmis/2022/ex/bgrd/backgroundfile-225283.pdf
There are several very good reasons:
1. It has barely been used for over a decade and not at all for 3+ years and
2. Metrolinx is removing the connection to the main line rail @ the Don Yard because they are expanding it and building the Ontario Line. It is useless without this.
3. Even if it could be reconnected to the main line, it would cost ca $50 million to reopen it at modern standards and able to serve modern rail cars.
I like rail too but this is really 'dead' - the City seems to be intending to maintain the right of way as a pathway and wildlife corridor which is great.