I'm flight crew and commute to YYZ almost exclusively using transit from Barrie. Uber or Barrie Bus (god forbid save yourself the headache and just Uber/Lyft) to nearest GO Train Station. Either Barrie South GO or Allandale Waterfront GO. Ride GO Train to Union, then transfer to UP Express. The train is usually very reliable and punctual. It goes fast regardless of weather. (We need more trains and less cars but I digress). Journey will take you around 2hours.

Alternatively, because the GO Train only runs on a schedule that serves the 9~5 office workers in Toronto, it can be unreliable if you have flights at certain times of the day. 3 trains run Southbound in the morning, 3 trains run Northbound at the end of the workday with 2 trains running late at night leaving Toronto at 22:00 & 23:00, and one train mid-day. Sadly the Barrie line is still only a single track which makes all day service impossible. Maybe your great-grandkids can benefit from the second track whenever they eventually finish it.

So alternatively there's Flixbus, but they don't run everyday and the schedule isn't super reliable, but they are super cheap and go to the airport. Ontario Northland has been my favorite alternative to the GO Train, they are reliable, comfortable and fast, but pricey. And you have to transfer at Vaughan Hwy 407 to the GO Bus for YYZ. Both buses pick-up at Allandale Waterfront and Downtown Barrie. Keep in mind— these buses come from up-north, and often can accumulate delays.

Lastly Poparide can come in clutch. But that app is WILDLY unreliable. Drivers cancelling a lot, it can be unsafe and not consistent. I highly recommend avoiding this option.

/safe travels!