The Toronto Transit Commission (TTC) Line 1 YongeUniversity is Toronto’s longest subway line, with track circuits utilized for signaling. Over the years, the number of daily passengers has greatly increased, and as the system aged the need for upgrading became readily apparent. A requirement of TTC was that the upgrade activity must not interfere with the operation of this busy commuter line, and that the new signaling system must function independently of the existing system. It would provide CBTC fallback functionalities, and work as an overlay to the current track circuit-based system.