What is the purpose of the trench being dug alongside the Stouffville Go track just north of Toronto?

Question

North of the city limit, there is a large trench being dug and reinforced with concrete (I think). It looks like it might be a subway tunnel. I'm curious about the future of public transit in the Greater Toronto Area, and I'd like to know if the trench is in fact a transit project. It certainly looks like one.

Answer 1

The link below has a list of current Go projects in metro Toronto area.