Impossible? No; Unlikely, Yes.
For a host of reasons.
Aside from those already discussed by
@Urban Sky and others..........
The Summerhill 'station' is not owned by the government, its privately-owned and would have to be purchased.
It would not be a cheap asset.
The LCBO makes a lot of $$ at that site, there are huge numbers of bulk buys at that store by the resto industry.
The cost of physically modifying it back to a railway terminal, while protecting its heritage are not immaterial.
It is not physically connected to the subway system currently.
You would also be running the service on CPKC's main line, that will be the case anyway for a distance of several km further east; but adding several more km which would require new/widened bridges and embankments, some of that abutting the homes of the richest people in the country, is not a small matter either.
Nothing that can't be overcome; but its not quite the easy low-lying fruit you might think.