I personally would not want Go to incrementally expand out into Peterborough. The future of Go should be a regional rail, closer to a metro than "train" just as the Paris RER is. Ontario definitely needs it's own interurban provider, but Go should not fit the bill. Go needs to stay within the GGH so it can be specialized in serving metro-style regional rail, and politically staying within the GGH while under the jurisdiction of Metrolinx.
Hah, $300 billion worth of infrastructure for Canada in the 21st century? The country probably needs at least $200 billion in infrastructure right now. I suppose that we have differentiating views on how Canada will evolve this century. I thoroughly believe that Canadians will step up and see how great this country could be, and rise to near-US level on the backs of immigration and exploding foreign markets. Of course, this will take tonnes of political will, but the pieces are all there in the puzzle, and we're at a huge advantage when compared to almost any other country on the world. We have the capacity to hold more immigrants, both politically through being one of the least racialized countries in the entire world, and by the fact that we're the 2nd largest country in the world. Canada's population could probably pass 100 million within 50 years, and Canadians could still not see the disadvantages. Our economy would be one of the most powerful in the world, and it would be a more vibrant country to live in, something to set us apart from anywhere else in the world.
So if that were to happen, the GGH might have a population of 20 million. The country would maintain it's density hugging the US border, and we could potentially get Eastern US/European type density patterns evolving in the southern prairies, and very dense rural areas in Southern Ontario. That'll all require tonnes of transportation infrastructure, revamping train routes, building roads (though hopefully we'll be able to build the country in a green, sustainable, human-oriented way too,) airports, ports, and so on. I could see why one would have a view that Canada might peak 50 million by the end of this century, but I think that we need to broaden our horizons and see what the country can really become.
More on-topic though, that Oshawa-Peterborough thing would be much better served by VIA, or some kind of post-VIA monster that actually provides a good service. Even at it's current density, Southern Ontario and Quebec could warrant European-style Rail with express HSR, regional trains, and trains servicing all those tiny little towns. While that possibility gets dismissed by pessimistic politicians that believe we don't deserve anything good, that'd be a great route for VIA to serve in it's current state. Correct me if I'm wrong, but VIA actually served Peterborough not too long ago, did it not? Maybe Ontario could fix up the track and donate some provincial money to keep the service going. I'm just worried because the way I see Go, it really doesn't fit the bill for that kind of service.