You guys are losing me on this whole need to tap-off (in context of a co-fare).
If I transfer from the GO train, I tap off at the Downsview Park GO platform (unless it is a default trip), and then I tap back in at the TTC turnstyle. Presto computer recognizes I am making a transfer between systems, only charges me the co-fare. I take the subway to York U (or anywhere on the TTC) and don't need to tap off.
In reverse: I tap on at York U turnstyles, get charged the regular fare. I get to Downsview Park, walk out of the TTC fare zone, tap on to the GO platform. The Presto recognizes a transfer, charges me the base GO fare (the fare to get from Downview Park to the next GO station) minus the difference between TTC regular and co-fare.
For example, let's assume Downsview Park was the same fare zone as the York U GO Station is now, the TTC co-fare was the same as it is on MiWay ($0.75), the TTC regular fare for Presto is the same ($2.65) and I travel bewteen York U and King City.
Inbound trip:
* Tap onto GO platform: +$4.82 (King City to Maple)
* Tap off at Downview Park: $5.49 - $4.82 = +$0.67
* Tap into TTC; no tap-off required: +$0.75
* Total fare: $4.82 + $0.67 + $0.75 = +$6.24
Outbound trip:
* Tap on at York U TTC; no tap-off required: +$2.65
* Tap on at Downsview Park GO: $4.77 - ($2.65 - $0.75) = $4.77 - $1.90 = +$2.87 (Downsview to Rutherford GO - TTC regular fare + TTC co-fare)
* Tap off at King City GO: $5.49 - $4.77 = +$0.72
* Total fare: $2.65 + $2.87 + $0.72 = +$6.24
So I don't see where this need to tap-off the TTC is coming from. This is the same way I understand it to work when I transfer between MiWay and GO. Am I missing something?