Not to mention that a subway is only a half-solution for Scarborough commuters.
They may see a couple of minutes shaved at the Scarborough end, but when they reach Yonge/Bloor, it's the same 5-10 minute wait until they can actually get onto a train to get to their job. The Scarborough subway doesn't address the Y/B chokepoint.
The billions* saved with an LRT can be put towards the DRL, which addresses the second half of the problem for Scarborough commuters. That's why, even if it doesn't fully fund the DRL on its own, the Scarborough LRT is a full solution.
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* Subway: $910M more upfront
+ cancellation costs
+ construction cost overruns (100% paid by Toronto taxpayers)
+ yearly maintenance in perpetuity (100% paid by Toronto taxpayers)
+ yearly operations in perpetuity (100% paid by Toronto taxpayers)
+ interest on 30-year loan
= billions more than the LRT, over 30 years