Located in the city of Vaughn, on the northern edge of Toronto, CN's MacMillan Yard is the carrier's Ontario nucleus.  It's large classification yard is where many trains destined for points throughout North America are assembled.  Visible from Regional Rd. #7 (formerly Provincial Hwy. #7) there's lots to see (but bring binoculars - the yard seems to stretch to the horizon).  Unfortunately, an overabundance of hydro wires and poles makes for difficult photo opportunities.
Image:  A look at the Yardmaster's tower and offices complex