|Locale||Greater Toronto Area|
|Daily ridership||15,000 (2014)|
|Opened||September 7, 1982|
|Line length||49.6 kilometres (30.8 mi)|
|Track gauge||1,435 mm (4 ft 8 1⁄2 in) standard gauge|
|Operating speed||90 miles per hour (145 km/h) (max)|
Stouffville is one of the seven train lines of the GO Transit system in the Greater Toronto Area, Ontario, Canada. Its southern terminus is Union Station in Toronto, and its northern terminus is in Lincolnville in Whitchurch-Stouffville. There are bus connections from almost every station to Toronto Transit Commission and York Region Transit services.
On weekdays during peak periods, trains operate twice per hour over the entire route, but in the peak direction only. Outside of peak periods, trains operate hourly in both directions between Unionville and Union, with a small number of trips covering the full line to Lincolnville. GO bus routes 70 and 71 provide service in the directions, time periods and segments not covered by train service. However, buses to and from Union Station bypass all other stations within the City of Toronto.
Between Union Station and Scarborough Station, the Stouffville line shares tracks with the Lakeshore East line. Most Stouffvile Line trains operate non-stop through the shared segment, but certain peak-period trains stop at Danforth Station and one train per day also stops at Scarborough station.
The track was originally laid by the Toronto and Nipissing Railway, and came into operation in 1871. The T&N merged with the Midland Railway of Canada in 1882. Only two years later, the Grand Trunk Railway leased most of the lines in the area as part of a major expansion plan, and purchased them outright in 1893. The Grand Trunk would later merge with the Canadian National Railway in 1923. CN would provide passenger rail service on the line until the formation of Via Rail in 1977.
On September 7, 1982, Via service was discontinued and replacement service—then just a single weekday run—was started by GO Transit.
On December 13, 2007, the government of Ontario announced funding to Metrolinx for network expansion, which included $20 million to build a second track to enable all-day two-way service between Union Station and Markham.
On September 2, 2008, the line was extended northwards from Stouffville to Lincolnville.
On February 2, 2015, select trains began stopping at Danforth GO Station as part of a year-long pilot project to increase GO service within the City of Toronto. As of June 2017, many peak-period trains continue to stop at Danforth station, and one daily train also stops at Scarborough station.
All-day weekday service was introduced on June 24th 2017, with hourly service in both directions between Unionville and Union Station during the midday and evening. Service in the counter-peak direction continues to be operated by buses.
The Stouffville line operates entirely over railways owned by Metrolinx. The line begins at Union Station and follows the Kingston Subdivision (GO Lakeshore East line) to Scarborough Junction, just east of Scarborough GO Station, with some trains stopping at Danforth Station along the way. It then branches north onto the Uxbridge Subdivision, originally the Toronto and Nipissing Railway.
In June 2013 GO held a first "Public Information Centre" of an environmental assessment study for expanding rail service in the Stouffville Corridor. This represented the completion of the second of five stages of work to implement expanded service, and recommended adding double track segments and other improvements between Union Station and Unionville to support increased train service levels.
Double tracking of the line from Unionville GO station to Scarborough GO is scheduled to begin in 2015. This section is proposed to become part of the SmartTrack line proposed by Toronto mayor John Tory.
On April 16, 2015, the Ontario government announced a Metrolinx initiative to increase rail service throughout the GO Transit network over the next decade, known as GO Regional Express Rail. Under the plan, diesel Stouffville line trains would operate every 20 minutes from Lincolnville to Union Station during peak periods, and hourly or better from Mount Joy to Union Station. In addition, electric trains would operate every 15 minutes between Unionville and Union Station.
In June 2015, new stations were approved in Toronto at Finch Avenue East and Lawrence Avenue East, to be built alongside the RER electrification. 
There are long-term plans to extend service beyond Lincolnville to Uxbridge, over tracks already owned by GO Transit. Until such an expansion, Uxbridge is served by a GO bus stop at Uxbridge station, and the only rail service north of Lincolnville is the York-Durham Heritage Railway.
- "Regional Express Rail" (PDF). Metrolinx. 5 September 2014. Retrieved 2 September 2014.
- "Stouffville line schedule" (PDF). GO Transit. 24 June 2017. Retrieved 25 June 2017.
- Daniel Garcia; James Bow. "GO Transit's Stouffville Line". Transit Toronto. Retrieved February 22, 2015.
- Gray, Jeff (2007-12-13). "Ontario to deliver on $100-million it had promised for public transit". The Globe and Mail. Retrieved 2007-12-13.
- Stouffvile Corridor Rail Service Expansion Class Environmental Assessment Study PUBLIC INFORMATION CENTRE #1|date=2013-06-18|accessdate=2014-02-16
- Richard Gilbert (25 February 2015). "Construction of second Stouffville track rolls forward". Daily Commercial News. Retrieved April 2015.
A tender was issued earlier this month to expand and improve 17 kilometres of corridor on a section of the Stouffville line between Scarborough Junction to Unionville GO Station.Check date values in:
- Ontario improving GO Transit service along all corridors
- "GO Regional Express Rail 10-Year Program: New Stations Analysis" (PDF). metrolinx.com. Metrolinx. Retrieved 21 June 2016.
|Wikimedia Commons has media related to Stouffville line.|
- GO Transit: Stouffville GO Train & Bus Service Schedule
- Daniel Garcia and James Bow, GO Transit's Stouffville Line
- "Stouffville Corridor Rail Service Expansion - Environmental Study Report" (PDF). R.J. Burnside & Associates Limited. Metrolinx. July 2014. Retrieved 19 September 2014.