The lower loop of the Yonge-University-Line was closed over the Thanksgiving Weekend, so that crews could install signal upgrades along underground track.
The current signals are nearly 60 years old. The outdated system is controlled by TTC operators, with an average three-minute gap between trains.
The TTC's Jessica Martin told CBC News that when the new computerized signal system is fully installed, the organization will be able to run more trains through the system.
But it will take at least another five years to complete the change to the new system.
Over the long weekend, the TTC arranged to have a series of shuttle buses replace the subway service between downtown stations.
Click on the video above to see a full report from the CBC’s Shannon Martin.