I'm honestly surprised they haven't ever done it yet. Even in the tight-headway 2019 timetable, the first eastbound train arrived at Union at 07:00, which gives it plenty of time to return westbound to Milton in time to start the 08:27 eastbound departure. Once peak service returns to pre-pandemic level, running one or two counter-peak trains could be a good way to help some Route 21 passengers avoid traffic congestion while also using train equipment more efficiently. At the moment it's not really an issue, since there's still equipment to spare.
Notes:
- In practice the counter-peak trains aren't necessarily the same physical trains as the peak direction trains. The peak direction trains may actually run through to another line while the counter-peak Milton trains arrive run from another line.
- Currently only Kipling, Erindale and Streetsville have more than one platform, so counter-peak trains would only be able to stop there, plus Milton. But that still seems fairly useful. Currently the triple-track segment ends about 400m west of Cooksville station. Extending it eastward through the station would be a nice way to add that station to counter-peak services while also reducing its impacts on CP.
- All of the stations have room for additional tracks,
as I illustrated in an earlierpost in this thread.