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For a brief period in 1966, the Toronto subway routes were interlined. At the time, it was considered a feat of engineering, as there is an underground, freeway like interchange between the lines. Interlining was discontinued because when a single train broke down, it paralyzed the entire system.
The underground interchange still exits, and was pressed into revenue service for a brief period during track and tunnel construction (I'll have to look up the dates). During the period of construction, transfers between the Bloor and University-Spadina trains took place at Museum Station instead of the usual St. George Station. Source of the graphic image: Wikipedia, credit for the enhanced image to David Arthur.