Don't be so sure, what everyone here forgets is that its 15 minute MINIMUM frequency all day.
The lakeshore lines already have service almost every 7 minutes at times during rush hour. Go look at the schedules.
The rush hour trains are already packed even with the 7-10 minute frequencies.
A lot of people here get confused with 15 minute... its 15 minute MINIMUM.
Yup, very important distinction. There will likely be layered service overtop of the "full" GO RER routes, which could be SmartTrack or a similar implementation, which means that in Toronto the minimum level of service could very well be 7.5 mins (2 overlapped 15 min frequencies). Get 2 branches overlapping with each other, and you've got subway-level frequencies, even off-peak.