In the late 19th century, there were a number of plans to connect the Parry Sound area to the Ottawa Valley (and from there, the Saint Lawrence River) to the east, Toronto in the south, and James Bay in the north.

[2][3] The Toronto–Parry Sound segment was completed in 1906, terminating in the south at Rosedale Junction,[4] running northward to the east of Lake Simcoe before crossing over the former Northern Railway of Canada (NRC) line to North Bay, which would later become known as the CN Newmarket Subdivision.

Historically reliant on generous federal loans to support the construction of new lines, one by one, the major railways became insolvent.

Late that year, an Order in Council instructed the CGR and CNoR to merge operations under a new name, the Canadian National Railway.

The integration of the Canadian Northern and Grand Trunk systems resulted in two parallel north-south mainlines in Central Ontario for the new Canadian National Railway: one passing to the west of Lake Simcoe through Newmarket, Barrie, and Orillia on its way to Nipissing Junction, and another passing to the east of Lake Simcoe on its way to Parry Sound; the two lines crossed each other at Washago.

[4] Around the same time, the Washago Connection was constructed, which replaced the level crossing of the two lines with two junctions and a short joint section.

[6] Centralized traffic control (CTC) had been installed along the line from Don to Richmond Hill in the previous year.

[4] By 1966, passenger services were being entirely routed through the Newmarket Subdivision south of Washago, including the Super Continental, which had a scheduled travel time from Toronto to Parry Sound of 4 hours and 5 minutes.

The service operated daily, leaving northbound from Toronto in the morning after southbound commuter trains had been cleared from the line, and returning to Union Station in the evening or sometimes late at night.

This forced the remaining through passenger services along the Newmarket Subdivision south of Washago to relocate to the Bala Subdivision, namely the Ontario Northland Railway's Northlander and the Via Rail Canadian, the latter of which subsequently adopted a route similar to the pre-1960s Super Continental, and initially had a scheduled travel time of 4 hours and 15 minutes to Parry Sound as a limited express with only one intermediate stop, at Washago.

Starting in the early 2010s, Metrolinx, the provincial Crown agency which oversees GO Transit operations, began to invest more heavily in the line.