[4][5] The line ran entirely within the eastern district of Scarborough, encompassing six stations and 6.4 kilometres (4.0 mi) of mostly elevated track.

In 2016, in order to free up funds for another transit project, the city reduced the extension to include only one station,[6][7] which was set to be completed by 2026.

[8] In 2019, Progressive Conservative premier Doug Ford reinstated the three-station Scarborough subway extension and committed to completing it by 2030, with all construction costs to be borne by the province.

[18][19] In October 2013, the TTC announced plans to give the lines official numbers to help riders and visitors to navigate the system.

In 1972, the Government of Ontario announced the GO-Urban plan to build an intermediate capacity transit system across suburban Toronto, particularly in Scarborough and Etobicoke, using the experimental Krauss-Maffei Transurban.

The Ontario government, in charge of GO Transit, was looking for a test site for the ICTS system and demanded that the TTC use it for one of their planned streetcar projects, selecting the Scarborough extension.

The TTC initially refused to make the change, arguing it was both the wrong solution and that since the construction of the line had already commenced this would be a waste of money.

[24] Ontario wanted to develop and promote its new technology, which had been designed for a proposed GO Transit urban service known as GO-ALRT.

Changes to federal railway regulations had made the new system unnecessary for GO, so the government hoped to sell it to other transit services in order to recoup its investment.

[27] The original "RT" logo was no longer featured on the trains, except when the Line 3 shrink wrap is removed but not re-applied yet.

[32] Immediately following the derailment, replacement bus service was implemented initially by shuttle buses serving the closed stations along Line 3.

Unlike the shuttle service, route 903 did not serve the closed Lawrence East, Ellesmere, Midland and McCowan stations.

[33] On November 30, 2023, the consulting firm Systra submitted a report to the TTC that found that, prior to the derailment, maintenance procedures on Line 3 were weak or non-existent and that track inspection staff lacked experience to understand how various defects could create an operating risk.

[34] On October 4, 2024, EllisDon began work to demolish and remove the Line 3 train tracks between Eglinton Avenue and the north end of Ellesmere station in preparation for constructing a busway.

The business proposal initially bore little fruit—a proposed pilot project in Hamilton was cancelled after meeting widespread public opposition, and the only other transit systems to use the technology, named the Intermediate Capacity Transit System (ICTS), at the time were Vancouver's SkyTrain and the People Mover in Detroit.

After Bombardier took over UTDC, it redesigned the technology with newer, longer cars, used to expand the SkyTrain network and also for new installations across the world.

The linear induction motors also allowed the cars to climb steeper grades than would be possible with traditional subway technology since wheel slip was not an issue.

In June 2024, the Detroit People Mover transit system announced the purchase of 12 Mark I trainsets (24 railcars) and equipment from Line 3.

The 302 originates at Bingham Loop, where it connects with the 322 Coxwell bus that travelled to the west, as well as Route 324 Victoria Park that runs north.

[51][52] After studying the revitalization and expansion of Line 3 in 2006, its replacement with alternate transit (light rail versus subway) became a subject of debate in the late 2000s.

(According to transit advocate Steve Munro, the need to rebuild the tunnel was discovered after the $190 million upgrade estimate was made.

[60] Extending Line 2 Bloor–Danforth, either along the current route or along a different alignment directly to Scarborough Centre station, was not considered cost-effective or justifiable.

Metrolinx issued a letter to Toronto City Council indicating it would cease work on the Scarborough portion of the line, because its position strayed from the original LRT agreement.

[6] Subsequently, the cost estimate for the one-stop subway extension increased to $3.2 billion, leaving the Crosstown East LRT unfunded.

[69] After Council's vote, Byford admitted that the cost estimates for "delays" was unnecessary,[70] which assumed the LRT's completion would be in 2026, the same date as the subway option.

However, according to Michael Warren, a former TTC chief general manager, the LRT could have been completed in 2020 at a cost of $1.8 billion, an estimate not presented to City Council when it voted.

[71] There was also the issue of whether there would be space for both expanded GO service and an LRT north of Kennedy station; however, Metrolinx subsequently denied there would be such a problem.

"[73] Mayor Tory was concerned that switching from subway to LRT would delay transit improvements in Scarborough, and might not get support from senior levels of government.

[15] By August 2023, newly elected mayor Olivia Chow had promised to build the busway, which was estimated to cost $55 million.

The busway was planned to remain in service after the Scarborough Subway Extension opened in order to provide extra rapid transit.