A cellular service map of the Toronto subway
Back in October of last year, smartphone addicts like myself got very excited at the prospect of the TTC installing WiFi on station platforms across the system. Plans were in place to outfit Bloor-Yonge and St. George stations sometime in late 2013 as a trial run, but the project has been delayed on account of reluctance from Canada's major carriers to participate. (Correction: According to the TTC's Brad Ross, the pilot projects at St. George and Bloor-Yonge are still a go for late 2013. Good news)!
Despite the inauspiciousness of the TTC's WiFi contract, subway riders aren't completely out of options when it comes to cellular use in the system. The subway does come above ground in certain places and there are other less-known opportunities to grab a signal. Reddit user colonkay posted a map charting these spots earlier today. You'll note that it doesn't make up a particularly significant portion of the total subway route, but it's a handy thing to keep in the back of your mind.
Worthy of note is the green dot near the centre of the map. It represents the expanse over the Don Valley between Castle Frank and Broadview stations where you can briefly grab a signal. I'd add to this the far east end of St. George Station's platform on the YUS line, where you can often get a couple of bars. Check out the Reddit thread for more hidden cell spots on the TTC and add your discoveries in the comments.
Map by colonkay