According to the recent USEP documents that were posted here earlier this week, a bridge above Yonge Street connecting to Union Station is under consideration, this bridge would connect to various potential platform extensions, providing Union Station a direct connection to Yonge Street.
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Here’s the official blueprint of the bridge included in the USEP documents:
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It can also be partially seen in the 3d model of Union Station Metrolinx showed in one of their social media posts a few months ago:
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Alongside this bridge over Yonge Street, a west bridge coming out of the Skywalk and UP Express station is also under consideration, and would also connect to future platforms extended out of the west side of Union, here’s the blueprint for the west bridge and a screenshot of it in that same Metrolinx 3d model video:
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The photos of the 3d model are very low quality and in awkward angles, but that’s just how Metrolinx posted the video.