"You can't get there from here." VIA Rail Canada operates the Niagara Falls, Ont. (or Windsor) to Toronto stretch and they don't accept AGR. Next best thing is to go to Niagara Falls, N.Y. and take VIA or Greyhound from there. Personally, I've wondered about just walking over the border and boarding in Niagara, Ont. -- foot crossings are generally pretty quick.
However, you can book Amtrak-operated services to Vancouver and Montreal with AGR points.