There are a number of places where a third track would be a challenge, such as at Eglinton or the CN York Sub (Snider Junction) where existing structures only support 2 tracks and it would be a big job to expand them. But building the second track would be a breeze, there's plenty of space and most of the infrastructure is already there to support it. All structures south of Aurora, except the Yonge Street overpass, are built for at least two tracks, with many built for three, such as the new bridge over the Don River south of Langstaff, completed last year:
Image of new Don Valley bridge from October 2016
Here's a
quick aerial overview I did of the structures along the southern Barrie Line:
View attachment 113789
I think the most likely segment to be triple-tracked would be from just north of Sheppard / Downsview Park station (which was designed for 3 tracks) to just north of Eglinton / Caledonia Station (which is being designed for 2 tracks). This would allow peak-direction local trains to pull off to the side at Downsview Park station to be overtaken by express trains. Meanwhile counter-peak service would be infrequent enough that express and local services could coexist without overtaking along the line.