That'd be great for links through the southwest of the horseshoe. I wonder if there's any room for rail there?
Go should also be providing an interurban service through Niagara, which is in desperate need of new interurban services, and all the rail infrastructure is there.
I agree that Peterborough is the most secluded from any of the GGH cities/towns by far, but I think that a rail link might make it more included. I honestly think the biggest barrier is a lack of a real fast link from the city to anywhere else in the GGH, other than Northumberland County. Peterborough-Clarington and the 401 = 30 or 40 minutes. Peterborough to Markham = Over an hour
Peterborough to Toronto = Around 2 hours
If Go could get a train to Toronto that takes about an hour, stopping at Agincourt to link up with the Midtown and Stouffville Lines, then going down Stouffville to Union, it could very well help Peterborough, and take almost 100% of all Peterborough-Toronto commuters as well as Peterborough-Markham commuters. It could also help Peterborough's industries, which would now have the population of the GTA to employ.