What I have never known (and is not explained in that video) is how far east/west of Queen and Yonge did they actually build that streetcar tunnel (if at all) and while I "get" that the decision at the time was to build B-D rather than continue with Queen why at no time since has someone said..."mmm, wonder if we should go back now an revisit that idea of running part of the Queen streetcar line under ground".
Cost is likely prohibitive but if, say, either/or/both of queen car or king car ran undgerground from, say, Jarvis to Bathurst it sure would move more people more efficiently than they do now.