That's actually an argument for buses, not streetcars. A route's frequency is determined based on the TTC's loading standards, which are about 50 people per bus, or 130 people per streetcar. So assuming that the ridership is not strongly correlated with the vehicle type, a bus would be more than twice as frequent as a streetcar for a given route.
That said, I do also like the idea of a Parliament streetcar as a blue-sky idea, as long as it's more than simply an electrification of the existing Parliament bus. I think as a first phase, it could be extended down the existing Cherry Street line to Distillery Loop, providing a new north-south connection for the developing West Don Lands. As a future phase, it could be extended under the railway into the Portlands along Cherry Street, connecting with the new East Bayfront streetcar at Commissioners Street along the way.
Existing tracks in blue, new tracks in bright green. Rosedale Valley bridge shown in blue to highlight the location of the existing unused subway structure.
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To reach Parliament Street from Cherry Street, I've proposed a one-way looping, westbound on King Street and eastbound on Queen Street. This is because Sumach Street isn't wide enough north of King to accommodate two-way streetcar traffic. A new westbound right turn streetcar track would be required at King & Parliament.
The big opportunity I see here is the available subway level of the Rosedale Valley segment of the Prince Edward Viaduct. As we all know, the bridge was built in 1918 with the foresight to include with a lower level for subway trains, which proved extremely helpful when the Bloor-Danforth Subway was built 48 years later. But the new subway only used the Don Valley segment of the bridge, since the curve would have been to sharp for subway trains to reach the Rosedale segment. Instead, a new covered bridge was built just north of it, leaving an unused subway level on the Prince Edward Viaduct, perfectly aligned for a Parliament streetcar heading to Castle Frank Station.
Rosedale Valley bridges: Prince Edward Viaduct (Bloor St) in foreground, Rosedale Valley Bridge (Line 2) in background.
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To accommodate single-ended streetcars underground at Castle Frank, it would take a big underground loop, which would be extremely expensive. But by the time this would open, we would have completed the transition to pantograph operation, so the next generation of streetcars could be bidirectional if we so choose. That would allow for a much cheaper-to-build arrangement at Castle Frank:
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