The Don Branch is a five-kilometre stretch of currently unused rail that starts at the CPR Leaside Train Yard. Warning: the train yard itself is active and police patrolled with no trespassing permitted.
The railway proceeds south to the Queen Street bridge overpass, where the Don Station terminus was once located. However, there is a significant stretch of railway starting just south of the Prince Edward Viaduct where the abandoned railway begins to merge with the heavily used CN Bala subdivision line.
There is also a Bayview Avenue overpass called Half-Mile Bridge, located near Evergreen Brick Works. Some people cross the bridge. However, there are no railings to hold and large gaps between the railroad ties. It's not for the faint of heart or those afraid of heights.
For a history of the railway see the Torontoist article referenced in the External Links.
In 2007, GO Transit purchased the Don Branch and, according to a 2016 article in Transit Toronto (referenced in the External Links), the Don Branch may soon be used as part of GO Transit's future Midtown Line.
There are many ways to access this railway. The address information provided here is one location where it can be accessed.