I really don't understand why are we even discussing the needs of a downtown garage in 2025. Unlike NYC or Montreal, the primary surface transit node are streetcars.
The 19 Bay itself operates with a maximum of 4 buses in the afternoon. Gone are the pre-pandemic days where a couple dozen buses flock the route.
The 65 Parliament operates with 6 buses max.
The 75 Sherbourne operates with 10 buses max.
The 94 Wellesley operates with 9 buses max.
The 97C Yonge branch operates with 3 buses.
The 114 Queens Quay East bus operates with 8 buses max.
The 121 Esplanade-River operates with 4 buses max.
Add the seasonal 202 Cherry Beach with 3 buses and throw in 20 buses for streetcar shuttles, it sums up to 67 buses. Add 20% spare ratio and it's up to 90 buses.
TTC wants to build a big garage with 250-300 buses. Building one downtown would mean 200+ buses are going to deadhead out of downtown traffic as they are not even needed there. That's a BS about of wasted resources to operate the garage on top an expensive BS proposal. Even if TTC builds a small garage, they still need to build another garage elsewhere. So this discussion is frankly quite naive and not well thought. TTC could use a new HQ more than a new downtown garage.
I think you need to think more broadly, in geographic terms.
There is no operating garage in the entire old City of Toronto. Every last one was shuttered, Danforth, Eglinton (at Yonge), Lansdowne, and Parkdale.
The result is a massive deadhead time, not just for the core routes, but most services in the core east end, in Don Mills, and a many other places.
The absence of a garage closer to the core is part of the reason for anemically slow response times to subway shutdowns that require shuttles.
As noted in posts above, a new garage is not necessary, Danforth is available to reactivate, with or without heavy repair functions. There is also plenty of space at the former Eglinton Site (Yonge) though that will surely be developed, plans are on hold for now, and much of the former garage space is there, along with a vast empty lot.
Drum's suggestion about a better use for the Commissioner's site could also make sense.
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As a separate, but related issue, the TTC's inefficient operation of buses creates an artificial suggestion of the need of more (rather than differently located) garage space.
The TTC should not ever need to operate more than 11 garages in the City. Queensway could be enlarged/replaced, but in general, the TTC wastes about 25% of the fleet through lousy operation/service planning and bad choices by Transportation services.
Add deadhead time to that for a bit extra.