You know, as someone who works with many Scarborough types who rely on transit, a subway to Scarborough honestly makes the most sense. These poor folks have to spend 2.5-3 hours on transit, first one or two buses to Vic Park stn, then B-D line, then another bus or two north to manufacturing areas near the airport. Totally nuts and easily solved by one or two subway lines along Sheppard and perhaps Eglinton, and maybe even Steeles?
Remember, for every office worker commuting downtown, there's probably five low income earners commuting crosstown to retail and manufacturing jobs. So while taking a streetcar home along Queen or King W/E may be charming to the Bay St office worker, someone who makes much less walking around a huge manufacturing/big box retail facility all day may think otherwise.