![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjQ4-h92xIz7rt65JLQTAjQiakuBQmW2PUbtEys-AY2DCDFq2kE4yMe124QYBRjgTDfDqYrG4f4pRFXthutHvRvTVEgdTNMSsL-cc_b9IDG3mpgKMDwIwfnH6D5QRad7gFAUqdH5vLcyqCl/s320/650px-TTCSubwayInterlined1966.svg.png)
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.