Regular service is expected to resume on Line 1 Tuesday morning after a massive delay Monday evening — but the TTC says it’s still being cautious.

Line 1: Repairs effected. Regular AM service will resume at St Clair West. — TTC Service Alerts (@TTCnotices) September 25, 2018

Crews worked overnight to repair a significant signal issue that caused delays up to 90 minutes along Line 1 southbound from Eglinton to Union and northbound from Union to St. Clair West.

The TTC says crews will continue to monitor the line closely as service resumes.

The issues started just before 4 p.m., as the rushhour commute was just starting.

Several people took to social media to express their frustration over the significant delays.

Line 1: Customers will experience significant delays southbound between Eglinton and Union and northbound between Union and St. Clair West due to an ongoing signal issue. — TTC Service Alerts (@TTCnotices) September 24, 2018

TTC Spokesperson Brad Ross tweeted the delay was caused by a significant signal issue at St. Clair West called a “track down,” which means the signals believes a train is occupying the track when it is not and signals fail safe to red. This caused trains to pass through the area very slowly, causing a back up and slow travel times.

He added the issue could not be fixed until service concludes overnight so riders will continue to be affected until trains stop running.

We have a significant issue with the track at St Clair West, as anyone traveling north knows. Despite best efforts, it cannot be repaired until tonight when service concludes. /1 — Brad Ross (@bradTTC) September 24, 2018

Ross said, “While we are upgrading the signal system on weekends to avoid and remedy issues like this, we’re not there yet. The existing system still requires maintenance and is still prone to failure.”

CORRECTION: This is a corrected story. An earlier version incorrectly stated the delays began around 7:30 p.m.