Ironically, see the Sheppard subway line, which has twice been on the bring of being shut down permanently due to low ridership. A tram line would've sufficed.
Except that that is rather misleading.
The line which was always intended to be Sheppard West/Dowsview to STC, with phase 1 going to Victoria Park was truncated.
Willowdale station was cut entirely.
Because of the above and not intermediate stop from Leslie to Don Mills, a competing/supplementary bus service remained, and the stations weren't even complete visually to spec, missing trackside wall finishes, custom lighting and ceiling slats in many locations.
Notwithstanding the above, the service has generally run at every 5M or so, and has been full in rush hours and moderately busy off-peak.
Certainly a subway was more than the density in place in 1988 required, however, the intent was always for the line to generate greater density over time, which, it has, in spades.
A tram line would be running service every 2 minutes and packed to the gills now.
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In general, I favour the R.C. Harris school of infrastructure building. He didn't build Line 1, though was certainly involved in its conception, or needless to say, Line 2. But when he had the Prince Edward (Bloor) Viaduct built, he had a deck for a subway put underneath to save money in the future should a subway be needed (Danforth was a dirt road then), the City was barely 500,000 people at the time.
He also built the Water Treatment Plant that now bares his name, the design was approved when the City was a mere 300,000 people........he built it with capacity to serve 800,000.
Much cheaper to over-build somewhat (that doesn't mean run maximum service on day one) .....rather than under-built and have to re-build.
Putting the east end of Line 5 underground will be a hugely disruptive project, in addition, Eglinton will have been wrongly sized when the surface tracks are no longer required, and much more expensive to maintain.
The western side had to be delivered first in order to reach a yard.
So that's all that should have been done in phase 1. The eastern side should then have been built in 2 phases, entirely underground to Victoria Park, then on to Kennedy.