Where are all those riders going using the SRT current hub as it not to Don Mills or Downtown Toronto???
		
		
	 
With great respect Drum the subway's justification is not people already riding transit today for existing trips, that might count for 20% of opening day ridership.
The object is clearly to take people off the 401 for comparatively local trips (within the City of Toronto) and to make some trips possible that are not practical today, as well as to support significant new development.
The Sheppard subway is full in rush hour, which is remarkable for a short-haul, 5 station route.   But it has done everything asked of it, and more, which is to say it has spurred on significant new residential development near everyone one of its stations.
But more, far more, is coming on the existing route,  at Don Mills Station alone there are proposals exceeding 5,000 units in the offing, another 3,000+ near Bessarion and Leslie, thousands more at Bayview and up Bayview (Tyndale Green) that will feed into the existing line.
If the TTC ran six-car trains, as they will, the development on the existing route would see them full at current frequencies within 15 years.
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The extended line, before factoring in for new development will connection to a major employment node at Victoria Park, provide a like to Stouffville GO, but most importantly, in the west, will line link to Downsview, set to be a huge node in its own right, but also creating a relatively quick link to York University from the east.
Add to that, this will vast levels of suburban intensification just as it has on the existing corridor.
Very conservative modelling suggests a ridership increase of ~200% on opening day.
	
		
	
	
		
		
			There are numerous LRT systems in the US that do not meet the threshold for it in the first place that sees less than 10,000 riders a day. Only have to go to Buffalo and Detroit to see them. Washington DC streetcar line open in 2016 2 years late that sees 800,000 free riders a year and will be scrap in 2027
		
		
	 
This is not a good comparison Drum.
Lets start with Line 4 annual ridership:  ~15M or about 50,000 daily boardings.   About 18x the example above.
But perhaps more importantly, daily ridership per km is ~3,000;
For comparison, the entire London Tube only hits 10,000 (per km);  the average per km daily ridership for the Washington DC subway ~3,300.
For a stubway......that's not so bad.
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Lets add a direct comparison, another stubway.   The Rockaways shuttle in NYC.    Its almost the exact same length as the current line 4, is serves only 9,000 daily riders.