Line 1 Yonge–University is a rapid transit line of the Toronto subway. It serves Toronto and the neighbouring city of Vaughan in Ontario, Canada. It is operated by the Toronto Transit Commission, has 38 stations and is 38.4 km (23.9 mi) in length, making it the longest line on the subway system. It opened as the "Yonge subway" in 1954 as Canada's first underground passenger rail line and was extended multiple times between 1963 and 2017. As of 2010, Line 1 was the busiest rapid transit line in Canada, and one of the busiest lines in North America. In 2022, it averaged over 670,000 riders per weekday.

The line forms a rough 'U' shape, with two portions running generally north–south that meet at Union in the southern part of the city's downtown, and then gradually spreading farther apart as they proceed northward. From Union station, the eastern portion of the line runs straight under or nearby Yonge Street, sometimes in an uncovered trench, for 16 kilometres (9.9 mi) to its northeastern terminus at Finch Avenue, connecting with Line 2 Bloor–Danforth at Bloor–Yonge and Line 4 Sheppard at Sheppard–Yonge. This eastern portion, often just called the "Yonge Line", serves Downtown Toronto, Midtown Toronto and York Mills before ending at Finch Avenue, the northern edge of North York Centre.

The western portion snakes northwesterly from Union, initially running straight under University Avenue and Queen's Park Crescent to Bloor Street, where it turns westerly to run under Bloor Street for about 700 metres (0.43 mi). Along this stretch, it interchanges with Line 2 at St. George and Spadina stations. At Spadina Avenue, it turns north to run for roughly 1 kilometre (0.6 mi) under Spadina Road before curving northwesterly to continue along the Nordheimer and Cedarvale ravines to the foot of Allen Road at Eglinton Avenue. It reaches the surface and continues northward in the road's median for 6 kilometres (3.7 mi) past Wilson Station, after which it resumes travelling underground and runs northwesterly on an off-street alignment below suburban industrial areas and the York University campus until Steeles Avenue. From there, it turns to parallel Jane Street for roughly 1.5 kilometres (0.9 mi) until its northwestern terminus in the neighbouring city of Vaughan's planned downtown core, the Vaughan Metropolitan Centre, at the intersection of Jane Street and Highway 7. This western portion serves the Annex and Forest Hill neighbourhoods in Old Toronto; Humewood–Cedarvale in the former York; Yorkdale–Glen Park, Downsview, the York University Heights–Northwood Park areas in the former North York; and the Vaughan Metropolitan Centre in Vaughan.

The line's name has been changed as it has been extended. Following its opening between Union Station and Eglinton Avenue along Yonge Street in 1954, it was called "the subway" (Yonge subway is its retronym). In 1963, it was extended along University Avenue to St. George station and renamed the "Yonge–University Line". Briefly in 1966, the Yonge–University subway ran in two branches: one west along Bloor to Keele station (Yonge–University–Bloor), the other east along Bloor and Danforth to Woodbine station (Yonge–University–Danforth) via Bay Lower station.

In 1978, the "Spadina" section was opened and the line became the "Yonge–University–Spadina Line" (YUS). Although only two stations are on Spadina Road, a larger portion of the line was originally intended to follow the planned Spadina Expressway, which was partially built as Allen Road. The subway also had an additional internal route number: route 602.

Unofficially, subway lines were already numbered, but in October 2013, the TTC announced plans to display line numbers publicly to help riders to navigate the system. In March 2014, the line was officially numbered and renamed "Line 1 Yonge–University", with the Spadina part being dropped from the name. Announcements, documentation and rapid transit maps across the system now refer to the line as "Line 1" or "Line 1 Yonge–University".

There were several early proposals to build a subway along or near Yonge Street, many of which involved running streetcars in a tunnel. Here are some of the proposals.

• In 1909, an English company offered to build and operate a subway along Yonge Street from Eglinton Avenue to Front Street. The plan was abandoned because the city would take over public transit in 1921, and the company's franchise would then terminate.
• In 1910, when running for mayor of Toronto, Toronto Star co-founder Horatio C. Hocken proposed building a "tube" along Yonge Street from north of St. Clair Avenue to Front Street. He dropped the idea after losing that election.
• In 1911, a city engineer planned a line from Bay and Front streets to Yonge Street and St. Clair Avenue. The electorate rejected the plan.
• In 1931, City Controller Hacker proposed a north–south subway running from Avenue Road and St. Clair Avenue south to Front and York streets, making a wide loop via Front, Scott, Victoria and Gerrard streets. The TTC rejected this proposal saying there was insufficient population to justify such a project.
• In 1942, the TTC proposed a north–south line under Bay Street from Union Station to Bloor Street then jogging over to Yonge Street to continue to north of St. Clair Avenue. This idea was rejected in favour of a subway completely along Yonge Street.

During World War II, workers travelling from their homes in "northern Toronto" (which would now be considered the downtown core) to the industrial areas to the east and west of the downtown area on Yonge seriously strained the existing road and streetcar networks. There was concern that the expected post-war boom in car ownership would choke the city with traffic. The scheme was first proposed by Toronto Transportation Commission in 1942 to relieve congestion, which was delaying their bus and tram services. The TTC formed a Rapid Transit Department and studied various solutions between 1942 and 1945.

A plan was put to the voters on January 1, 1946. The plan had two parts. First, it featured a "rapid transit subway" operated with subway trains from Eglinton Avenue to the north as far as College Street to the south. The line would continue directly under Yonge and Front Streets to Union Station. Second would be a "surface car subway", diverting streetcar services off Queen Street and Dundas Street. This would run mostly along Queen Street, with each end angling north to reach Dundas Street west of Trinity Park and Gerrard Street at Pape Avenue. The route would run directly under Queen Street from University Avenue to Church Street, with the rest off-street. The vote was overwhelmingly in favour, and Toronto City Council approved construction four months later.

The plebiscite contained the condition that the federal government would subsidize 20 percent of the project. The federal Minister of Reconstruction, C.D. Howe, promised federal support in an October 3, 1945, letter. However, the funding fell through over a disagreement about the details of the employment arrangements. A scaled down proposal, about 20 percent smaller, was agreed to in its place. The work along Queen Street was abandoned temporarily, and the original $42.3 million ($722 million in 2023) was reduced to $28.9 million ($493 million in 2023) plus $3.5 million ($59.8 million in 2023) for rolling stock. After a two-year delay due to postwar labour shortages, construction on the new subway did not start until September 8, 1949. A total of 1.3 million cubic metres (1.7 million cubic yards) of material was removed and some 12,700 tonnes (14,000 tons) of reinforcing steel and 1.4 million bags of cement were put into place. A roughed-in station was constructed below Queen station for a proposed Queen line, but that line was never built.

Service on the Yonge route would be handled by new rolling stock, and the TTC was particularly interested in the Chicago series 6000 cars, which used trucks, wheels, motors, and drive control technologies that had been developed and perfected on PCC streetcars. However, the United States was in the midst of the Korean War at the time, which had caused a substantial increase in metal prices, thus making the PCC cars too expensive for the TTC. Instead, in November 1951, an order was placed with the Gloucester Railway Carriage and Wagon Company in England for 104 cars for $7,800,000 ($88.8 million in 2023) including spare parts.

The Toronto Subway typeface and TTC logo were also designed during this period. The logo used during the subway's development was designed by mid-century architect John C. Parkin and chief architect Arthur Keith. Against the wishes of Walter Paterson, the chief engineer, TTC chairman William McBrien and general manager H.C. Patten rejected the design in favour of one that was more similar to the one previously used on TTC vehicles.

After five years of construction, Ontario Premier Leslie Frost and Toronto Mayor Allan Lamport officially opened the 7.4-kilometre (4.6 mi) long Yonge subway on March 30, 1954. It was the first subway in Canada.

The original Yonge Street subway line went from Union subway station near the namesake railway station north to Eglinton station. Dignitaries, including the premier and the mayor, rode the first train that morning, going north from the yards at Davisville station, and then from Eglinton station south along the entire line. The line was then opened to the public, and that day at 2:30 pm, the last streetcar made its final trip along the Yonge streetcar line.

Trains operated at average speeds of 32 kilometres per hour (20 mph). The plan to operate two-car trains during off-peak hours was abandoned in favour of four-car trains, and six-car trains were standard during most periods, with some eight-car trains used during peak periods.

On February 28, 1963, an extension was added to curve north from Union Station, below University Avenue and Queen's Park to near Bloor Street, where it turned west to terminate at St. George and Bloor Street.

On February 26, 1966, the Bloor–Danforth line opened, from Keele to Woodbine. For six months, as a trial, the Yonge–University line operated as two branches, Eglinton–Keele, and Eglinton–Woodbine. The interline was determined not worthy, and the Yonge–University line was cut back to St. George on September 4, 1966.

On June 23, 1969, the University subway service from St. George to Union stations was discontinued entirely after 9:45 p.m. from Mondays to Saturdays and all day on Sundays and holidays. The 5B Avenue Road buses run in place between Eglinton and Front Street whenever the University subway did not operate, with side-jaunts to St. George station to capture passengers from the Bloor subway. This arrangement remained in place until January 28, 1978, when the Spadina subway opened north to Wilson Station.

On March 31, 1973, the line was extended north from Eglinton to York Mills, and on March 29, 1974, to Finch. These two extensions were part of the North Yonge Extension project, bringing the subway to North York.

Stations were also planned for Glencairn (between Eglinton and Lawrence, though another Glencairn station would be built later on the Spadina section), Glen Echo (between Lawrence and York Mills) and Empress (between Sheppard and Finch, later opened as North York Centre station).

On January 27, 1978, the Spadina segment of the line was opened, going from St. George station, the north terminus of the University line, to Wilson station. From St. George station, the 9.9 km (6.2 mi) segment ran north and northwest to Eglinton Avenue and William R. Allen Road, then north along the median of the Allen Road to Wilson Avenue. This extension had been proposed as part of the Spadina Expressway, but when the expressway portion south of Eglinton Avenue was cancelled after massive protests, the subway was still built following the route through Cedarvale Ravine. Hence, it was called the Spadina line, though it follows Spadina Road for less than 2 km (1.2 mi).

On June 18, 1987, North York Centre station was added between Sheppard and Finch stations as an infill station.

On August 11, 1995, at 6:02 pm, the Russell Hill subway accident occurred as a southbound subway train heading toward Dupont station crashed under Russell Hill Drive, killing three passengers and sending 30 to hospital. This accident prompted the Toronto Transit Commission to review its practices and put resources into safety.

On March 31, 1996, the Spadina segment of the line was extended 2 km (1.2 mi) from Wilson station north to Downsview station (renamed Sheppard West in 2017). At the time, a newly elected provincial Progressive Conservative government cancelled its share of funding that would have extended this route northward to York University and Steeles Avenue. However, this extension was later constructed and opened on December 17, 2017.

As of November 17, 2016, with the Presto fare gates installed at Eglinton station, all of the stations along this line are Presto-enabled.

On December 17, 2017, the western portion of the line was extended 8.6 kilometres (5.3 mi) north to Vaughan Metropolitan Centre station in York Region, via York University. The Toronto–York Spadina subway extension (TYSSE) project built six new stations at a cost of $3.2 billion, with 6.2 kilometres (3.9 mi) in Toronto and 2.4 kilometres (1.5 mi) in York Region. The TYSSE was the first new section of a subway line to be opened since the opening of Line 4 Sheppard in 2002. Upon opening, most stations on the extension saw below average ridership compared to the rest of the subway system.

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The line is mostly underground but has several surface sections between Sheppard West and Eglinton West, and between Bloor–Yonge and Eglinton. Most of the line between Bloor–Yonge and Eglinton stations was originally constructed in open cut, with the short section between St. Clair and Summerhill stations having since been covered over. Evidence of this can be seen in the tunnel: there are no columns or walls between tracks, and ballast and drainage ditches are present, something not seen in the rest of the subway system. There are also tree stumps and the stubs of lamp posts in the tunnel. There are also clues outdoors: seemingly unnecessary railings along the sides of a nearby street, which was once a bridge over the tracks, and empty lots following the trains' right-of-way marked with signs warning heavy vehicles and equipment to keep off because they might fall through to the columnless tunnel below.

Most of the tunnel was constructed by a cut-and-cover method, but some sections were bored, as noted below. All stations, whether by transfer or fare-paid terminal, connect to surface TTC bus and/or streetcar routes. Other surface and train connections are noted below.

Since 1996, TTC stations have been built or modified with elevators, ramps and other features to make them accessible to all. As of January 2022, 30 stations on the line are fully accessible. All stations on the line will be made accessible by 2025, as per the Accessibility for Ontarians with Disabilities Act.

Because the line opened in sections from 1954, it has a relatively high number of crossovers, which were mostly constructed at terminal stations to turn back trains. There are 17 diamond crossovers located between the service tracks along the length of the line. There are also eight storage tracks, which can also be used for reversals. The high number of possible turnbacks gives the TTC more flexibility when planning maintenance or in the event of an emergency service disruption.

The original design of the oldest stations in the subway system, which are on the Yonge line (from Union to Eglinton), are mainly utilitarian and characterized by vitreous marble wall tiles and the use of the Toronto Subway typeface for station names. Eglinton Station is the only station to retain this wall treatment, though Queen Station retains a narrow band of original blue Vitrolite tiles near the ceiling at platform level.

The design of the stations on the University line was mainly utilitarian and this style (sometimes referred to as "bathroom modern") was later used for Line 2 Bloor–Danforth as well. Queen's Park and St. Patrick stations have circular and semi-circular cross-sections because they are constructed in bored tunnels. Museum station was renovated in the late 2000s to have columns that resemble artifacts found in the nearby Royal Ontario Museum.

Lawrence, Sheppard–Yonge (formerly Sheppard), and Finch stations are similar to each other in design, but have different colour schemes: Lawrence is red and cream, Sheppard is yellow and dark blue, and Finch is light grey, medium grey, and dark grey. York Mills station formerly followed the same design scheme—in light green and dark green—until it was renovated.

The section of the line between Spadina and Wilson stations (formerly the Spadina segment) opened in 1978 has art and architecture that is unique for each station, such as flower murals in Dupont station or streetcar murals in Eglinton West station. The art originally installed at Glencairn and Yorkdale stations had been removed, as the former's art had faded in sunlight and the latter was too costly to operate. Since late 2017, work is underway to restore the art in both stations, with Glencairn's being fully re-installed in 2020, albeit modernized.

North York Centre station is an infill station. Its design is different from the other stations in the original North Yonge extension. Sheppard West station, which was opened in 1996 as Downsview station, has art and architecture that is different from the stations built earlier along its western portion of the line.

Stations on the 2017 extension from Sheppard West north to Vaughan Metropolitan Centre, in keeping with the pattern of the original Spadina line, also feature public art and architecture from notable creators. However, the platform walls have no tiles or other cladding and are simply bare concrete, though structural elements on the platforms themselves are clad, as is the case with much of the Line 4 Sheppard stations.

On the Yonge portion of the line, nearly all stations located at cross streets are named after said streets, while on the University portion, they are either named for local landmarks with the cross street subtitled below (e.g. Osgoode – Queen Street) or after cross streets but with a "West" suffix for stations at streets that have counterparts along Yonge, though Dundas West station is on Line 2 Bloor–Danforth. The pattern of using landmarks as station names was exclusively used on the original (southern) section of the University branch, and the West designated street-naming convention is typically used on the former Spadina (northern) section. The two interchange stations on the University branch where it intersects Line 2 Bloor–Danforth are named St. George and Spadina after the north–south cross streets of Line 2, which runs below Line 1 between these stations. Due to various factors, some stations along the Spadina portion are named, formerly were named, or are proposed to be renamed using landmark or district names, albeit without subtitles: the stations at Steeles Avenue and Highway 7 (which have no corresponding stations along the Yonge branch) are respectively named "Pioneer Village" (after the nearby Black Creek Pioneer Village, which has since been renamed the Village at Black Creek) and "Vaughan Metropolitan Centre" (after Vaughan's new downtown core, based on the precedent set by North York Centre and Scarborough Centre stations). Sheppard West was originally called "Downsview" but was renamed in 2017 to avoid confusion with the adjacent new Downsview Park station, and Eglinton West will be renamed "Cedarvale" (after the Cedarvale neighbourhood to the south) when it becomes an interchange station with the opening of Line 5 Eglinton in the fourth quarter of 2024.

As a result of the George Floyd protests, Toronto City Council indicated in 2020 they intended to rename Dundas station because its namesake, Henry Dundas, delayed the British Empire's abolition of slavery in the 1700s. As of 2024, however, the Dundas name has yet to be changed.

Southbound station platform signage on both branches indicates Union as a terminal station due to it being located at the southernmost point of the line's rough 'U' shape, where it turns northward when travelling along either branch. The train destination signs display the northwestern terminal station as "Vaughan" rather than its full name, Vaughan Metropolitan Centre, for brevity. Until the 1990s, train destination signs read "VIA DOWNTOWN" after the terminal station name.

As with other TTC subway lines, Line 1 operates most of the day and is generally closed between 2:00 a.m. and 6:00 a.m. EST on weekdays and Saturdays, and between 2 a.m. to 8 a.m. on Sunday. Trains arrive at stations every 2 to 3 minutes during peak periods and every 4 to 5 minutes during off-peak periods.

During the morning peak, from 6:00 am to 9:00 am Monday to Friday, half the trains are turned back at Glencairn station resulting in limited service north of that point. The turnback was moved from St. Clair West station to Glencairn station in 2016, and plans called for it to be moved farther to Pioneer Village station in December 2017 when the Line 1 extension opened.

Overnight service on the Yonge segment of the line is provided by 320 Yonge Blue Night from Queens Quay to Steeles Avenue with headways of 3 to 15 minutes. The University segment does not have an overnight service.

Line 1 is operated using only the TTC's Toronto Rocket (TR) subway trains, which are based on Bombardier's Movia family of trains. Unlike other trains in the Toronto subway rolling stock, the TR trains have a "six-car fixed" articulated configuration with full-open gangways, allowing passengers to walk freely from one end to the other. The TR trains were scheduled for delivery starting between late 2009 and early 2010, but was delayed until late 2010 due to production problems. They entered revenue service on this line on July 21, 2011, replacing the older H5 and the T1 series trains, which had been used on this line. (The T1 series trains, which used to operate on this line from 1995 until 2015, were transferred over to Line 2 Bloor–Danforth where they replaced the older H4 and H6 series trains).

From the line's opening in 1954 until 1990, it was operated with G-series cars, and was also served with a mix of M1 and H1/H2/H4 subway cars between 1965 and 1999.

Between 2021 and 2022, the TTC transitioned its Line 1 trains to one-person train operation (OPTO), which removes the secondary guard member – stationed at the rear end of the trains – who operated the doors. This change leaves only the subway drivers at the front to fill the dual role in 2023. OPTO went into effect between St. George and Vaughan Metropolitan Centre stations in November 2021, and was rolled out on the rest of the line effective November 20, 2022.

Gap trains are empty trains stored on pocket tracks and brought into service in a gap between full-route trains to relieve overcrowding. In October 2018, the TTC restarted the practice of using gap trains to relieve crowding at Bloor–Yonge and St. George stations, where respectively 225,000 and 135,000 passengers transfer trains daily. The TTC observed that one empty gap train can clear a crowded platform at Bloor–Yonge.

The TTC had previously run gap trains prior to late 2017 but had discontinued the practice because of a "change in operating philosophy". The practice was reinstated in response to a potentially dangerous overcrowding incident that occurred at Bloor–Yonge in January 2018.

In October 2018, the TTC used three gap trains, which sat on pocket tracks near Davisville, Eglinton and York Mills stations and moved into southbound service when station over-crowding was detected. For November 2018, the TTC planned to run a fourth gap train (which would sit on either the pocket track between Lawrence West and Glencairn stations or the pocket track between Eglinton West and St. Clair West stations) in the morning peak period plus another during the afternoon peak.

Gap trains can also increase the capacity of Line 1, which often runs above its scheduled capacity of 28,000 passengers per hour. To address that demand, the TTC normally runs an average of 25.5 trains per hour through Bloor–Yonge and St. George stations in the morning peak period. With three gap trains, it can run up to 28 trains per hour.

Starting in the late 2010s, the TTC began converting its fixed-block signal system to a moving block—based automatic train control (ATC) system on Line 1 Yonge–University at a cost of $562.3 million. Work to convert the line to use ATC was completed in September 2022.

The benefits of ATC are:

• a reduced headway between trains from 2.5 minutes to 2 minutes during rush hours, allowing a 25 percent increase in the number of trains that can operate
• fewer signal-related delays relative to the old fixed-block system
• a more efficient use of electricity, thus reducing operational costs
• allowance for single-track, bidirectional operation for trains in passenger service, albeit with reduced frequency, which allow for off-hour maintenance of the opposite track

In 2009, the TTC awarded a contract to Alstom to upgrade the signalling system of the existing section of Line 1, as well as equip its Toronto–York Spadina subway extension (TYSSE) into Vaughan, with moving block–based communications-based train control (CBTC) by 2012. The estimated cost to implement ATC on Line 1 was $562 million, $424 million of which was funded by Metrolinx.

The first section of the "Urbalis 400" ATC system on Line 1 entered revenue service on December 17, 2017, between Sheppard West and Vaughan stations, in conjunction with the opening of the extension project. On November 4, 2017, the TTC successfully completed a 13-day test of ATC with trains using it in regular service between Dupont and Yorkdale stations. At the conclusion of the test, the feature was turned off between these two stations to allow installation of ATC through the complex interchange at Wilson Yard.

ATC was permanently extended south to Dupont station on December 3, 2018; to St. Patrick station on May 12, 2019; to Queen station on February 24, 2020; to Rosedale station on November 21, 2020; to Eglinton station in October 2021; and finally to Finch station on September 24, 2022.

There was a phase 6 for fixes, improvements and enhancements, as well as an adjustment to ATC at Eglinton station to accommodate the shifting of the Line 1 platform north by 24 metres (79 ft); this was completed by May 14, 2023.

Prior to September 2022, ATC was operating on 79 percent of Line 1, and the partial implementation of ATC had resulted in improvements. Trains travelled between Vaughan Metropolitan Centre and Rosedale stations 3.5 minutes faster. Just prior to 2020, ATC allowed for an 8 percent increase in trains per hour, and the number of trains scheduled in southbound service in the morning peak at Bloor–Yonge station increased from 22 to 25.5 trains per hour. Signal violations decreased by approximately 50 percent from 2017 to 2020.

Converting all of Line 1 to ATC required the installation of 2,000 beacons, 256 signals, and more than 300,000 metres (1,000,000 ft) of cable.

As of 14 May 2023, the timetable for ATC conversion on Line 1 was:

Phase	Location	Completion
1	Yorkdale station to Dupont station	2017 Q4
2A	Vaughan Metropolitan Centre station to Sheppard West station	2017 Q4
2B & 2C	Wilson Yard interface (Sheppard West station to Yorkdale station)	2018 Q4
3A	Dupont station to St. Patrick station	2019 Q2
3B	St. Patrick station to Queen station	2020 Q1
3C	Queen station to Rosedale station	2020 Q4
4	Rosedale station to Eglinton station	2021 Q3
5	Eglinton station to Finch station	2022 Q3
6	Adjustments to ATC	2023 Q2

v t e Yonge North extension
Legend
Yonge Street Rapidway Storage facility High Tech Bridge Langstaff GO Highway 7 Rapidway Richmond Hill line Royal Orchard East Don River Clark CN York Subdivision Steeles York Region Toronto Cummer/Drewry (unconfirmed) Finch To VMC via Union
All stations are accessible

The Yonge North subway extension is a Metrolinx proposal to extend the Yonge Street portion of Line 1 north of Finch Station to Richmond Hill in York Region. As of 2021, the project entails building two underground stations plus two surface stations north of Finch station. One of the underground stations will be at Steeles Avenue, with the other located at Clark Avenue. From south of Highway 7, the extension would veer east away from Yonge Street, rising to the surface and then turning north along GO Transit's Richmond Hill line. The two northernmost stations – one between Highway 7 and Highway 407 (dubbed "Bridge station" by Metrolinx) and another 400 metres (1,300 ft) farther north at High Tech Road – would be built on the surface along the railway corridor. Bridge station would also be a hub for Viva and GO Transit buses as well as being connected to the existing Langstaff GO Station. The estimated cost of the 8-kilometre (5.0 mi) extension is $5.6 billion.

Originally, prior to 2021, the extension was to have had five new stops, all underground, located at Cummer/Drewry, Steeles, Clark, Langstaff/Longbridge and the Richmond Hill Centre Terminal of Viva bus rapid transit. In May 2017, the originally proposed 7.4-kilometre (4.6 mi) extension would have cost $5.6 billion. However, by 2021, the cost of this version of the extension had risen to $9.3 billion, resulting in a reduction of its scope.

The extension was projected to take a decade to build and would have replaced many of the 2,500 buses per day that run along the route. Ridership on the proposed extension was estimated at 165,000 per day. York Region estimated that the extension would help to create 31,000 jobs, and that the extension would carry 58 million riders annually by 2031.

On June 15, 2007, the Ontario government announced plans to fund this extension as part of a network of rapid transit growth called MoveOntario 2020.

In April 2009, the environmental assessment for the Yonge North subway extension was approved.

In November 2014, an addendum to the environmental assessment was approved to include an underground train storage facility and surface facilities.

By 2016, the province had committed to fund 15 percent of the preliminary engineering and design for the project.

On June 2, 2016, the provincial government gave $55 million to Metrolinx in order to work with the TTC and York Region on a detailed plan for the subway extension.

In May 2017, Toronto mayor John Tory announced he would not support planning for the Yonge North subway extension unless there was a funding commitment for the Relief Line to relieve crowding on Line 1, expected to be at capacity by 2031. If such a funding commitment had been authorized, city staff could have completed 15 to 30 percent of the design for both projects, including schedules and better estimates, by the end of 2019. As of May 2017, planning was more advanced for the Yonge extension than for the Relief Line, though the latter has since been replaced with the Ontario Line under the premiership of Doug Ford.

Metrolinx media
Map of extension as proposed in October 2023

By March 2021, Metrolinx had reduced the scope of the project in order to reduce the estimated project cost from $9.3 billion to $5.6 billion. The number of stations was reduced from six underground stations to two underground stations plus two surface stations. Two of the three originally proposed underground stations at Cummer Avenue, Clark Avenue and Royal Orchard Boulevard would be eliminated, saving $400 to $500 million per station, with only the station at Steeles Avenue being retained. The two northernmost stations, Langstaff/Longbridge and Richmond Hill Centre, would be replaced by the surface Bridge and High Tech stations. As well, the storage yard at the north end of the line would be built on the surface rather than underground. Markham City Council voted to reject this new route realignment including all surface portions of the route. This in effect cancelled the route option that the city council voted against. In July 2021 and January 2022 respectively, the province announced that the stations at Clark Avenue and Royal Orchard Boulevard would be retained; Royal Orchard station was to be funded by revenues related to the intensification of the surrounding area as a transit-oriented community.

The Yonge North subway extension consists of two primary contracts. The contracts are:

On April 27, 2023, Infrastructure Ontario and Metrolinx issued a request for qualifications (RFQ) for the advance tunnel contract, marking the first phase of procurement for the Yonge North subway extension.

This contract will include the design and construction of:

• A 6.3-kilometre (3.9 mi) tunnel from the existing Finch station to a portal south of Highway 407
• Design and construction of launch and extraction shafts, tunnels, as well as headwalls for stations and support of excavation for emergency exit buildings
• Design, procurement, and supply of tunnel-boring machines and tunnel liner segments
• Reinforcement and improvement of soil and works necessary to facilitate tunnelling under and next to the CN Railway tracks and the York–Durham sewage system
• Construction of CN right-of-way separation barriers to expedite safe corridor access and construction

To expedite construction, tunnelling will begin first, followed by a separate contract to build the stations, rail, and systems.

On December 1, 2023, Infrastructure Ontario (IO) and Metrolinx released a request for proposals (RFP) for the advance tunnel contract package. The three teams previously shortlisted through the RFQ process (CrossTransit Group, North End Connectors, and Toronto-York Tunnel Connectors) were invited to submit RFPs with proposals detailing how they will deliver the project. Upon evaluating the proposals received, IO and Metrolinx expect to award this tunnelling contract in late 2024.

As of Infrastructure Ontario's March 2023 update, there is no timeline identified for the procurement of the stations, rails and systems (SRS) contract.

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Toronto council approved the plan in principle in January 2009 provided there were upgrades to the existing line to support the additional ridership from York Region. Specifically, Toronto politicians stated that the Yonge line does not have the capacity to support an extension to Richmond Hill until a separate relief line is built between Pape and Osgoode stations. In the morning rush hours, southbound trains on the Yonge line usually reach capacity between York Mills and Eglinton stations.

In March 2016, York Region officials stated their belief that SmartTrack, electrified GO service, the Spadina subway extension and automatic train control would be implemented by 2026 and be sufficient to support the extra ridership of an extension to Richmond Hill. Thus, the deputy mayor of Richmond Hill wanted to start construction of the extension by 2019. Vaughan politicians and officials noted a June 2015 Metrolinx report which indicated the Yonge line would have enough capacity to carry the new demand if the Spadina extension and Regional Express Rail on the Richmond Hill line were implemented. However, Metrolinx has no immediate plans to provide electrified GO service on the Richmond Hill line and Toronto transit advocate Steve Munro noted that the line would be at 96 percent capacity in the peak hour and there would be some overcrowding due to variations in traffic over the hour.

In March 2016, the Toronto mayor's office said that SmartTrack and automatic train control may have also been required, which was reaffirmed by TTC spokesperson Brad Ross that June. This was affirmed again by Metrolinx in January 2019, thus reversing its opinion in its June 2015 report. The 2019 report also predicted that if both phase 1 of the then-planned Relief Line to Pape station and the Yonge North extension had been built, Line 1 would have been over capacity by 2041. At that point, the Relief Line would have needed to be extended farther north. However, the Relief Line was later replaced by the Ontario Line, which will run to Eglinton Avenue at Science Centre station.

Given the wait for the Richmond Hill extension, there were plans to construct Viva Bus Rapid Transit bus lanes along Yonge Street from Finch Avenue to Highway 7. However, by April 2014, the plan was scrapped in favour of only constructing the busway north of Highway 7. A group in York Region had lobbied for the plan's cancellation due to concerns about a loss of treed boulevards and private property when widening Yonge Street for bus lanes through Thornhill. The revised busway opened in 2020.

• MoveOntario 2020
• Yonge streetcar line – Predecessor to the Yonge portion of Line 1

• Official route page
• Toronto–York Spadina Subway Extension
• Yonge North Subway Extension at Metrolinx
• Yonge North Subway Extension at York Region Transit
• "Yonge Relief Network Study" (PDF). Metrolinx. June 25, 2015. Archived from the original (PDF) on March 13, 2018. Retrieved July 24, 2016.
• "Yonge Subway Extension – Environmental Assessment Submission and Project Update" (PDF). City of Toronto. December 16, 2008. Retrieved July 24, 2016.