It is operated by the Toronto Transit Commission, has 38 stations[5] and is 38.4 km (23.9 mi) in length, making it the longest line on the subway system.

[2] 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.

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.

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.

[21] 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.

[26] 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.

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.

[35] 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.

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.

[42] [43] 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.

[47] The TTC had previously run gap trains prior to late 2017 but had discontinued the practice because of a "change in operating philosophy".

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.

[48] The benefits of ATC are: 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.

[49] 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.

[54] 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.

[61] 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);[62]: 8–9  this was completed by May 14, 2023.

[64] 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.

[65][67] 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.

[71] 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.

[70] 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.

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.

[77] 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 – including CrossTransit Group (Vinci, Ferrovial, Sener, Janin Atlas), North End Connectors (Aecon, Dragados, Ghella, EXP, TYPSA), and Toronto-York Tunnel Connectors (Acciona, Bouygues, Hatch, Parsons) – were invited to submit proposals detailing how they will deliver the project.

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

[79] 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.

In the morning rush hours, southbound trains on the Yonge line usually reach capacity between York Mills and Eglinton stations.

[83][84] 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.

[citation needed] 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.