However, by 2012, the Toronto Transit Commission stated that a relief line will be required by the 2030s, given the overcrowding and high demand along the Yonge corridor.

[14][15] The Relief Line was included in the regional transportation plan The Big Move and was noted as one of Metrolinx's top 15 transit priorities.

Metrolinx prepared the plan for the Ontario Line in just three months based on a proposal by transit consultant Michael Schabas.

[29] The Globe and Mail reported that no previous plan had ever considered making Ontario Place a rapid transit destination and that the announcement surprised everyone, including mayor of Toronto John Tory.

The documents showed that the proposed route would be markedly different from that of the Relief Line South and involve significant lengths of at-grade or elevated track.

[30] In October 2019, Tory and Ford reached a tentative deal in which the city would endorse the line and the TTC's subway network would not be taken over by the provincial government.

[32] The Ontario Line project is being delivered through various public–private partnerships (P3), progressive design–build and traditional procurement contracts, which are all being staged accordingly for their successful delivery.

[34] On November 17, 2022, the contract was awarded to Connect 6ix (a consortium led by Hitachi Rail, Transdev, Plenary Americas, and Webuild) with a projected in-service date of 2031.

[34] On November 9, 2022, the contract was awarded to Ontario Transit Group (a consortium led by Ferrovial and Vinci), with a projected completion date of 2030.

[36] The contract included the design and construction of: On November 17, 2022, Metrolinx and Infrastructure Ontario issued an RFQ for the northern portion between Gerrard station and the Don Valley Bridge.

[37] On January 17, 2024, the contract was awarded to Pape North Connect (a consortium led by Webuild and Fomento de Construcciones y Contratas).

[39] The contract included the design and construction of: In addition to the above contracts, there will also be a series of early-works projects for bridge, track and other preparatory activities to help advance the delivery of the Ontario Line primarily at two locations: Exhibition GO Station to Strachan portal and Don Yard portal to the future Gerrard Station.

[42] A ground-breaking ceremony attended by Ford, Tory, and other politicians and officials was held on March 27, 2022, at Exhibition Place, despite no major contracts having been awarded at that point.

[44] Route 501 Queen streetcars were required to divert, with new tracks on Adelaide and Richmond Streets allowing a diversion due for completion in 2024.

The northeastern terminus of the Ontario Line would be Science Centre station at Don Mills Road and Eglinton Avenue.

The Ontario Line platform would be on an elevated structure above Science Centre station's bus terminal, located north of Eglinton Avenue on the east side of Don Mills Road.

Immediately south of this station, the line would turn west along the Hydro One right-of-way, and cross the Don Valley on a new bridge.

South of Gerrard station, the Ontario Line would run along the northwest side of GO Transit's Lakeshore East rail corridor located on an embankment.

After crossing the river, the line would pass the GO Transit Don Yard before descending into a tunnel just east of Cherry Street.

[55][59] Construction of the MSF would require the demolition of an Islamic centre and a shopping plaza that includes local businesses such as the popular Iqbal Halal Foods grocery store, serving the primarily Muslim population of Thorncliffe Park.

In December 2021, Metrolinx made a $49.5 million agreement with the Islamic Society of Toronto to move to a larger facility at 20 Overlea Boulevard.

The elevated Ontario Line station will be along Don Mills Road at the northeast corner of its intersection with Eglinton Avenue.

There is also a future proposal to extend streetcar service south along Broadview Avenue into the Port Lands in the vicinity of East Harbour station.

A second new entrance will be at the southwest corner of Queen and Simcoe Streets, one block west of University Avenue, where a bank building stands.

[79]: 25 In May 2022, Mayor John Tory and the Law Society of Ontario (co-owner of Osgoode Hall along with the Province of Ontario) objected to placing an entrance on a corner of the Osgoode Hall grounds as this would require the removal of a section of lawn, some tree cover and a section of the cast-iron fence that encloses the grounds.

The city preferred that space be taken from the northbound lanes on University Avenue, but Metrolinx said building a shaft there would conflict with Line 1 running below.

On February 21, 2023, the Ontario Divisional Court dismissed the last injunction, allowing Metrolinx to proceed with removing the trees the next day.

Ontario Line riders will be able to transfer to a 501 Queen or 510 Spadina streetcar at surface stops without having to fully cross a street.

Tail tracks will extend west of the platforms to Dufferin Street for storing and turning back trains and a possible future extension.

[43] In February 2022, the Ontario Ministry of Transportation (MTO) released the Connecting the GGH: A Transportation Plan for the Greater Golden Horseshoe to provide a 30-year vision for enhanced mobility within and across the Greater Golden Horseshoe (GGH), including future expansion plans for the Ontario Line.