Old documentation says that the track in that photo was a service track, not a siding. Note the split frog derail. Even in 1974, it wasn't being used as a passing track.
It was still in place in 1992 according to the
City's Aerial Photograph collection, but gone by 2002.
The line was OCS until about 2009ish.... so clearly no investment in capacity other than maintenance in all that time. (In 1974 the line had weight restrictions, but I haven't gone searching for when the track was beefed up to modern standards EDIT - that only happened post 1993).
Another good example of the "the rails are there so we must be able to add a train right away" fallacy.
As much as I cringe at some of the station overbuilding, I give ML credit for designing with the future in mind.... that scene today is different for a reason. Somebody finally considered what it would take to run a more frequent service, and built to that. The point being, GO (or VIA) (or CN) never intended to run more than a peak service on the line....until the last decade.
- Paul