Here's some short answers to alklay's questions.
1. Is it financially viable? 2. Who owns the tracks? Is transit on railway tracks even a good idea (and is it close enough to density modes)? 3. Why was it not carried out? 4. How does this differ from Go Transit? 5. And does Transit City make this redundant?
1. At least as viable as any other transit project. Remember, the Transit City manifesto starts by saying the city cannot afford and has no intention of paying for the new lines, but here we are one year later and the projects are underway. A DRL would quickly pay for itself in increased taxes.
2. It wouldn't be entirely in the rail corridor, particularly along Front Street, but when there's space available or negotiable in corridors, why not take advantage of it?
Looking just south of Bloor/Danforth, it runs through extremely dense areas filled with trip generators, especially between the CNE and the West Don Lands, where stations would be literally feet from CityPlace, the SkyDome, Convention Centre, ACC, the St. Lawrence 'hood, pretty much everything along Queen's Quay and, most importantly, the entire central business district. In the stretches where it turns up to the B/D line, it'd run through areas that are as dense as any other in the city, like Parkdale or Riverdale, not to mention intercepting the east/west downtown streetcar routes.
All of these areas are underserved by useful transit, and given the usual state of the Spadina or Queen streetcars, one could say these areas are not served by any transit at all.
3. It wasn't carried out for the same reasons hardly anything has been done for transit in Toronto in decades: bickering municipalities, lack of funding, etc. Jack Layton and others didn't want downtown to have great transit because they wanted development to occur in the suburbs.
4. It's a high frequency subway line from Pape (Greektown) to Union (downtown) to Dundas West (Roncesvalles) and possibly beyond, not a commuter train that only stops every 5-6 km. It wouldn't require additional fare, it would be a seamless addition to the subway network.
5. Transit City offers absolutely no transit improvements for downtown and could cause massively increased crowds on existing subway lines, especially Yonge. Clearly, a downtown line is desirable even with today's level of crowding. Future Transit City-related plans could include extending Don Mills and Jane streetcars to Union Station, but only a subway line would also be able to handle the large crowds that would ride the line. Frankly, people will not switch from the Yonge line to streetcars that aren't grade separated, greatly diminishing the relief value of such a line.