In a way it's kind of good that the failure happened right before the PM peak, because the single-tracked lines (Stouffville and Barrie) don't have counter-peak service. So they can send out a bunch of trains at arbitrary times without being limited by the location of passing tracks to meet oncoming trains.
The schedules at this point are irrelevant (it sounds like they're just running to headways) but on paper this is the largest delay on a moving train that I've seen to date:
This is incredibly vague to the point of being misleading. The tweet should have mentioned that it was only in reference to the PM Peak service today. Here's the actual message from the GO website. 30 minute is the target peak direction headway and counter-peak (where applicable) should be 30-60 minutes
The service alert directs you to a link to see current departure information, but when I click it I get a message that there are no departures, which is clearly false because I can see trains departing on GO Tracker.
2 train per hour would be brutal for the Lakeshore lines given that they normally run 4 to 6 trains per hour during the PM peak. I would assume that for those lines it's a case of underpromise and overdeliver, since they need to run a lot of trains outbound anyway to get to their layover yards at the ends of the lines in order to start the morning service tomorrow. I hope they wouldn't just send out two local trains per hour and call it a day.
The Milton line is generally running on schedule because it is dispatched entirely by CP and TTR, not CN. The 16:55 departure from Union seems to have disappeared off GO Tracker, but via word of mouth I've heard it is actually running as per schedule.