Line 3 Scarborough

Line 3 Scarborough, originally known as Scarborough RT (SRT), was a medium-capacity rapid transit line that was part of the Toronto subway system of the Toronto Transit Commission in Toronto, Ontario, Canada. The line ran entirely within the eastern district of Scarborough, encompassing six stations and 6.4 kilometres (4.0 mi) of mostly elevated track. It was connected with Line 2 Bloor–Danforth at its southwestern terminus, Kennedy, and terminated in the northeast at McCowan. Until its closure in July 2023, the system had a ridership of 4,720,000 per year.

The rolling stock of Line 3 consisted of smaller, semi-automated, medium-capacity trains, rather than the larger heavy-rail subway trains used on other lines in the system. Designated by the Toronto Transit Commission (TTC) as the S series, these were Intermediate Capacity Transit System (ICTS) Mark I trains built by the Urban Transportation Development Corporation (UTDC). The trains were powered by linear induction motors and operated on 1,435 mm (4 ft 8+1⁄2 in) standard gauge tracks, unlike the heavy-rail subway lines and the Toronto streetcar system, which use the unique 1,495 mm (4 ft 10+7⁄8 in) Toronto gauge.

The line remained mostly unchanged from its opening in 1985 and contained two of the least-used stations in the system. Beginning in the late 2000s, Toronto City Council debated over competing plans to revitalize and expand the line, to convert its right-of-way for use by modern light rail vehicles, or to close the line and extend Line 2 Bloor–Danforth farther into Scarborough along a different route. In 2013, the council decided on a three-station extension of Line 2 to replace Line 3 along a different route. In 2016, in order to free up funds for another transit project, the city reduced the extension to include only one station, which was set to be completed by 2026. 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.

The TTC planned for Line 3 to cease operations in November 2023, with shuttle buses running in place of Line 3 train service until the Line 2 Bloor–Danforth subway extension to the existing Scarborough Centre station opened for service, which was estimated to be in 2030. However, a train derailment in July 2023 resulted in the line permanently closing four months ahead of schedule. By March 2023, a plan existed to convert a portion of the existing right-of-way between Kennedy and Ellesmere stations into a bus right-of-way, including an additional stop at Mooregate Avenue / Tara Avenue, located near a pedestrian bridge that spans over the former Line 3 and GO Transit's Stouffville line between Eglinton Avenue and Lawrence Avenue. In 2023, the busway was targeted for completion by 2025.

From when the line opened in 1985 until 2015, it was known as the Scarborough RT or SRT. The "RT" in Scarborough RT stood for "rapid transit". The name Scarborough Line was used on the official TTC website and 2014 TTC Ride Guide. In October 2013, the TTC announced plans to give the lines official numbers to help riders and visitors to navigate the system. The line was numbered 3, as it was the third rapid transit line to open in the system. New signage was installed in March 2014. In 2015, the name was simplified to Line 3 Scarborough.

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. However, KraussMaffei was forced to abandon development when the West German federal government declined further funding. GO-Urban then used some of the technologies from the Transurban to develop a simpler steel-wheeled version, the ICTS system.

During this period, the TTC had been working on plans to extend its own network with a series of streetcar systems using a new and greatly enlarged streetcar design, the Canadian Light Rail Vehicle (CLRV). 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. However, as the Ontario government was providing 75 percent of the funding for the line, they changed their minds when the government threatened to cut the funding.

At Kennedy station, there are clues revealing that it was originally built for streetcar operation; it is possible to see old low-level streetcar platforms protruding under the current high-level platforms, and the loop to turn streetcars proved too sharp for safe operation of the ICTS cars, which did not have a reason to turn around, so the loop was replaced by a single-track Spanish solution-like crossover. Ontario wanted to develop and promote its new technology, which had been designed for a proposed GO Transit urban service known as GO-ALRT, first proposed in 1982. 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.