The TTC used to do that back when the Bloor Danforth line first opened the subway was considered to be one zone across the city and needed two tickets to board if you entered it outside of downtown, for example Main Street Station had collectors both on the mezzanine level of the station rather then at the entrance of it where they are now, If you came in by bus you would have to deposit a ticket to get on the subway as well or if you came on from the street you would have to pay two fares. I'm not 100% sure if that's how it worked as it was stopped before I was born or at least before I started riding the subway. Personally I think zone fares work better for something like Go transit, although I think they should make all of Toronto be one zone and have the fare a little bit higher then the TTC maybe say like 4 dollars or something like that.