It should be possible to get two birds with one stone.There are almost 500 Peterboro residents who work in Toronto, plus another 400 working in Whitby and Oshawa. To put that in perspective, there are only 400 Niagara Falls residents working in TO.
Not arguing either way, rather making a comparison to another far flung area that did receive a GO rail connection. What I do think a priority should be going east is a GO service along the CP line to about Brock in Whitby (as mentioned at the top of the page). Makes sense. Three lines in Peel, three in York, but in Durham just one? That's insufficient.
Looking at the Havelock sub west of Peterborough: it runs parallel to Hwy 115 till the 115 joins Hwy 35 and veers south (https://www.google.ca/maps/place/Pe...x5037b28c72318e0!8m2!3d44.309058!4d-78.319747).
Looking at the CP Belleville sub as it runs east and passes Bowmanville: it veers north a bit (https://www.google.ca/maps/place/Bo...49991bfc2abf863!8m2!3d43.9126042!4d-78.688019).
A new track connecting the two CP lines would be about 12 km long, and there isn't much in the way to make that new track particularly costly. And then, Whitby / Oshawa / Bowmnaville / Peterborough could share the same rail service.
Say, two trains daily in each direction, running all the way to/from Peterborough. Plus, two or three peak-time trains running between Toronto and Bowmanville only, taking some pressure off the Lakeshore East line.
The CP Belleville service would be pretty useless for Pickering and Ajax (the line runs way too far north). So, it could run express from Agincourt in Toronto all the way to the new station in Whitby (perhaps at Brocks St or Annes St). Then, a couple of stations in Oshawa, plus one in Bowmanville, and then express to Peterborough. Not much of a delay / detour compared to staying on the Havelock sub, but substantially more riders served.
The other part of the puzzle is the Toronto terminus, and the route to reach it. Dupont; Summerhill; Union via the old Don branch; Union via the Uxbridge sub are the options on the table.