In their final report, the consultants suggested that the Prince Edward Viaduct, which spans the Don River Valley, should include a lower deck for a future subway.

[14] The lower deck was built, but the first plan for a line to use it was not made until June 15, 1933, when the TTC published a report which suggested construction of a subway and an expressway broadly following Bloor Street and Danforth Avenue.

[16] Before the subway was built, the Bloor streetcar line operated along the route between Jane Street and Luttrell Avenue (located just west of Shoppers World Danforth).

This was opposed by Etobicoke, Long Branch, Mimico, New Toronto and Scarborough, who wanted the project to be funded solely by the TTC.

The line was 12.9 kilometres (8.0 mi) long,[15] and ran about 20 to 40 metres (66 to 131 ft) north of Bloor Street and Danforth Avenue.

The solution was a change in political thinking, where the subway was seen as a subsidized public service, instead of a utility that needed to balance its books.

[36] This program would result in making the subway system more accessible,[37] add new bus and streetcar platforms,[38] and improve the connections to regional buses and GO trains.

[40] The line does not run under Bloor Street or Danforth Avenue, except at the Prince Edward Viaduct; otherwise, it is offset to the north by about a city block.

These modernizations include new and updated wall finishes, signage, lighting and public art, as well as the installation of elevators for accessibility needs.

[53] Plans to add a second exit for Donlands, Greenwood, and Woodbine stations were deferred in late February 2011 due to lack of funding.

[57][58] On December 1, 2019, all subway station collector booths were permanently closed and replaced by roaming customer service attendants.

[59] All Line 2 stations except Chester connect to surface TTC bus or streetcar routes during regular operating hours.

On Sundays, these routes operate through the early morning hours, because the subway starts service at 8:00 a.m. instead of the usual 6:00 a.m.[8] Frequency is 6 to 30 minutes.

[65] Due to the opening of the Bloor–Danforth line and the additional services that were required, a new set of trains were purchased from the Hawker Siddeley group.

In mid-2016, a few Toronto Rocket trains were used on Line 2 because of an air conditioning malfunction in numerous individual T1 cars, combined with a hotter than average summer.

This was after Toronto Mayor John Tory accepted a challenge posted on Twitter to ride an overheated T1 train on Line 2 during a hot summer day.

In 2017, the TTC planned to replace the T1 fleet with 62 new trains, possibly using a successor of the Toronto Rocket type from Bombardier to eliminate the time needed to prototype a different model.

[71] However, in March 2019, the TTC reversed its decision and planned to delay the purchase of new train sets by refurbishing the T1 fleet to extend its life by a decade.

[74] The RFI closed on September 18, 2020, and the TTC hosted an information session date on May 4, 2021, with potential suppliers to discuss the background, industry engagement, procurement model, and technical overview with interested manufacturers.

Due to its location next to the Canadian National Railway (and GO Transit Lakeshore East and Stouffville line) tracks, it was possible for trains to be delivered directly to the subway.

[86] During his 2010 mayoralty campaign, Rob Ford denounced the idea of light rail transit and instead proposed to replace the Scarborough RT with an extension of the Bloor–Danforth line.

[88] In June 2012, the idea of a Scarborough subway extension was a key part of Toronto's proposed OneCity transit plan.

[94][96][97] (However, most of the $1 billion saved was subsequently diverted back to the SSE to cover additional estimated construction costs for the one-stop subway.

[86] In February 2020, Metrolinx released a cost–benefit analysis showing that the $5.5-billion project cost for the three-stop extension is double the estimated benefits of $2.8 billion over 60 years.

[112] The northern terminus of the extension will be at Sheppard Avenue East and McCowan Road; it will have an adjacent TTC bus terminal as well as a pick-up and drop-off area.

[117] On March 10, 2020, Metrolinx and Infrastructure Ontario issued a request for qualifications (RFQ) related to advance tunnelling works.

On August 20, 2020, Metrolinx issued a request for proposals to shortlisted proponents, including Acciona, East End Connectors (Dragados, Aecon and Ghella) and Strabag.

[124] By January 2023, the TBM (named "Diggy Scardust" after Ziggy Stardust, David Bowie's stage persona and fictional character) had started digging at a speed of 10–15 metres (33–49 ft) per day.

[126] In February 2022, following the close of the RFQ process, Metrolinx and Infrastructure Ontario released an RFP to three shortlisted bidders: Dragados (including AECOM), KSX Integrated Design-Builders (Kiewit and SNC-Lavalin), and Scarborough Transit Connect (Aecon, FCC Construcción, and Mott MacDonald).

The scope of the SRS contract included the following: The TTC's Rapid Transit Expansion Study, published in 2001, identified three possible western extensions to the line.