I imagine they would also end up subsidizing a large amount of short trips on the subway around downtown to get to and from Union Station.
Yeah, the deal that Metrolinx has with the 905 agencies just won't scale with the TTC.
It works because:
1) Transit ridership is a relatively small proportion of trips to stations in the 905.
2) The co-fare subsidy is a lower percentage of the overall larger fares from 905 stations.
----- Burlington to Union is $9.99, Burlington Transit is $2.50 on Presto, so GO retains $7.99 after the 50c co-fare.
80% of a non-transit-connecting fare
----- Etobicoke North to Union is $5.02, TTC is $3 on Presto, so GO would retain $2.52 after the 50c co-fare.
50.2% of a non-transit-connecting fare
3) Only one transit co-fare has to be subsidized per trip.
----- A vanishingly small number of trips currently involve two co-fare eligible trips.
----- Cooksville to Union is $6.18. Miway and TTC are each $3, which would be $2.50 subsidy to each. Go would retain $1.18.
19% of a non-transit-connecting fare
And finally, neither GO nor the TTC are interested in increasing ridership in their critically overcapacity sections.
GO's capacity hits the highest points at the Toronto stations. Adding service in the 416 is expensive, and the trips that makes room for are lower value than the long-hauls from the 905.
The TTC is not interested in having large volumes of GO riders swamping the subway, and likely thinks that a mutually-shared subsidy with GO would just end up with similar revenue but greater ridership downtown where there is the least spare capacity.