Starting Tuesday, September 4, GO Transit will operate a new afternoon train, leaving Union Station at 3:18 p.m. Mondays to Fridays. The train serves Kennedy, Agincourt and Millken GO Stations, before ending its trip at Unionville GO at 3:55 p.m. At Unionville, the train will meet GO buses to take you to all Stouffville line stations north of Unionville.
Since it will be providing extra train service, GO will no longer operate the buses that currently leave the Union Station GO Bus Terminal at 3, 3:10 and 3:15 p.m. Mondays to Fridays for various points along Stouffville line.
Three morning trains will include two more railcars, providing 320 more seats each trip. Two of these trains will hae more cars, starting Tuesday, September 4; the third will take effect Tuesday, October 9.
You can view the new schedule for the line here.