|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 via Go Transit Buses, Toronto Transit Commission, and York Region Transit.
Trains run only during the rush hour periods with 7 trains running southbound in the morning and 8 running northbound in the afternoon and evening. One southbound and 2 northbound trains only travel as far north as Unionville with bus connections travelling the rest of the route. When trains are not running, stations are served by GO Bus route 71 which runs the corridor. However, with the exception of Union Station, it bypasses all stations within the city of Toronto.
Between Union Station and Scarborough Station, the Stouffville line shares tracks with the Lakeshore East line, but it currently does not stop at Scarborough. Effective February 2, 2015, select trains stop at Danforth GO Station as part of a year-long pilot project.
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 29 June 1998, GO Transit restored full service to the Lakeshore lines, and terminated service to Danforth and Scarborough on the Stouffville line—these stations were fully integrated into the Lakeshore East line route schedule.
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's northern terminus was extended northwards from Stouffville to Lincolnville.
In April 2011 GO representatives stated that ultimately GO would like to run an all-day service on the Stouffville line, however they described this as "in the distant future". As of April 2013 the Stouffville service includes six southbound morning trains and six northbound afternoon trains. Service to and from Unionville includes one additional southbound morning train and two additional northbound afternoon trains. 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.
A list of stations on this line (in order inbound to Union):
The line begins at Union Station and follows the 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 Metrolinx-owned Uxbridge Subdivision, originally the Toronto and Nipissing Railway. North of Unionville station, the route heads roughly north-east in the direction of the Lincolnville terminus.
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.
There are long term plans to extend service from Lincolnville to Uxbridge as GO Transit owns the line formally from CN Rail. Currently Uxbridge is served by GO buses with a stop at Uxbridge Station.
On April 16, 2015, the Ontario government is working with Metrolinx to have more train service along the Stouffville line, known as GO Regional Express Rail over the next decade. During peak hours, trains would run in peak direction every 20 minutes from Lincolnville to Union Station. During off peak hours, trains would run two-way hourly service or better, from Mount Joy to Union Station and electrification will be in place from Unionville to Union Station, with trains running every 15 minutes along the electrified line.
In June 2015, new stations were approved in Toronto at Finch Avenue East and Lawrence Avenue East, to be built alongside the RER electrification. 
- "Regional Express Rail" (PDF). Metrolinx. 5 September 2014. Retrieved 2 September 2014.
- 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.
- GO Transit outlines Stouffville’s future|date=2011-04-27|accessdate=2013-04-28
- Stouffville GO Train and Bus Schedule|date=2013-04-6|accessdate=2013-04-28
- 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.
- 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.