Several plans for an east–west downtown subway line date back to the early 20th century, most of which ran along Queen Street.

[3][4] The Relief Line was included in the regional transportation plan The Big Move and is one of Metrolinx's top 15 transit priorities.

After fully converting the line's fleet to higher-capacity Toronto Rocket trains, the section between St. Clair and Bloor–Yonge stations was no longer officially over capacity.

[11] The future implementation of automatic train control will help further increase the capacity of Line 1 to 33,000 people per hour per direction.

[10] Other factors are expected to reduce demand such as the extension of the University portion of Line 1 into Vaughan and other local transit improvements.

However, after factoring in population and employment growth and the proposed extension of Line 1 north into Richmond Hill, Metrolinx projects it will be at 96 percent of its capacity by 2031 even with committed improvements.

[8][12] Metrolinx in a separate report projects that with the implementation of GO Transit's GO Expansion (formerly known as Regional Express Rail service), passenger traffic at Union Station will double or triple 2005 volumes by 2031.

[19] On 25 August 1910, the first serious proposal for the Relief Line was made by Jacobs & Davies, a New York City‐based firm of consulting engineers, with The Report on Transit to the Mayor and Council of the City of Toronto.

In the 1980s, the TTC, Metropolitan Toronto and the Government of Ontario did several analyses of forecasted urban growth and alternative transportation scenarios for the downtown to Bloor area.

In 1982, the Accelerated Rapid Transit Study considered multiple options for a "radial line", connecting Dundas West and Donlands stations with a U-shape through downtown.

The third alignment considered ran along an elevated guideway on Parkside Drive at the eastern edge of High Park to Keele station.

In 2008, Metrolinx published The Big Move, the regional transportation plan for the Greater Toronto and Hamilton Area.

Metrolinx Chair Rob MacIsaac stated in 2008 that the line is unlikely to be brought forward from its projected 2020 start date but deemed it of "regional significance".

Metrolinx officials stated that capacity issues may allow the Relief Line to be given higher priority in the regional transportation plan, The Big Move.

[28] In February 2013, the Metrolinx Board approved changes to The Big Move that re-prioritized the eastern segment of the Relief Line to the 15-year plan, and made it one of the 15 top priority projects in the Greater Toronto and Hamilton Area.

[6] The Downtown Rapid Transit Expansion Study (DRTES) was completed by the TTC in 2012, which examined four alternative Relief Line configurations between Pape and St. Andrew, with varying extensions north to Science Centre station (at Don Mills and Eglinton) and west to Dundas West station.

[29][30][31] The TTC's 2015 DRL study identified four potential corridors, which involved combinations beginning at Line 2 at Broadview or Pape, and going through downtown via King or Queen Streets.

[33] On 1 June 2016, the provincial government announced $150 million funding for Metrolinx to plan and design the Relief Line.

[34] In late June of the same year, a Toronto Star article reported the estimated cost of Phase 1 with eight stops to be $6.8 billion; the project was unfunded.