Are you sure it was a full GO trainset? If so, I wonder if it was a trainset chartered by CP, though that seems unlikely. There are no direct connections to any other GO corridors east along the North Toronto Sub or the Belleville Sub, especially with the Belleville Sub bridge over the Don Valley out of service and fenced off.
Quite often you see a couple in the Runnymede Yards, often sitting right across the Runnymede underpass. It used to make me double take until I got used to it. Ditto for the new Locos, they often sat in the Runnymede Yards.
Glancing at the Star and they had their first interview with ML ne chief. Much revolved around RER and he made specific references to Hydrail and he gave me the impression that ML was SERIOUSLY considering it.
^ They are considering it in terms of evaluating how realistic it will be when/if private sector bidders include it. Phil Verster has previously said it's up to the private sector to determine the percentage it will be included and how much risk they are willing to take on.
I created a table listing the gaps between stopping trains at each station along the line. I included the Stouffville line schedule because some peak service to Danforth and Scarborough stations is actually provided by Stouffville Line trips.
During the midday hours, there is indeed 15-minute service along the LSE, apart from a few timing adjustments.
Top block is the schedule, bottom block is the time since the last stopping train at each station.
But there is nowhere near 15-minute-or-better service during peak periods in either direction, with many gaps as long as 40 minutes: