You know what, I think that just might work. Working from the
Stouffville Line timetable, here's what that run would look like:
0709 Depart Union
0724* Pass southbound train at Scarborough
0730 Depart Kennedy
0741 Depart Milliken
0743 Pass southbound train at siding
0746 Depart Unionville
0751 Depart Markham
0756 Depart Mount Joy
But here's the problem. If the train continues north, it will cross another southbound train before it gets to Stouffville, at a location where I don't think there's any passing track.
The only option at this point seems to be to terminate the run at Mount Joy, and become the 0759 southbound run which would have otherwise originated at Lincolnville. (With OCS, it's probably possible to move this trip forward to 0757).
The train which currently runs that service can depart Linconville later, as soon as track is available.
So it's possible to run a counter-peak train, but it would mean having the second southbound train skip Milliken, Agincourt and probably Kennedy stations, as well as creating a roughly one hour gap in service at Stouffville station at the peak of rush hour.
Though extending southbound service by one train somewhat offsets these losses.
*Rough estimate, since I don't actually know how much time it takes to get from Scarborough to Kennedy.