Full service has resumed on the Yonge-University-Spadina line after a radio communications system failure disrupted the morning commute, the TTC says.

Trains on the YUS line were brought into service later than normal because of the problem, so shuttle 140 buses were used on the entire subway line in an attempt to ease overcrowding.

At one point, the TTC warned that some trains were up to 30 minutes late, but the delay was eventually reduced to just a few minutes before the failure was resolved and full service resumed around 8 a.m.

Ross said 48 trains usually operate on the YUS line at the peak of the morning commute. At 7 a.m., that number was around 25, but it was quickly increased to 44.

It appears subway platforms weren't overly congested.

Many of the shuttle buses appeared to be empty and there didn't appear to be any lineups or crowds outside subway stations, CP24's Cam Woolley reported.

Trains were running as scheduled on the Bloor-Danforth, Sheppard and Scarborough RT lines while YUS trains were delayed.

The TTC warned riders about the YUS delay before the first trains were scheduled to depart around 6 a.m. The first warning was sent in a tweet at 5:48 a.m.

Cause of failure under investigation

Ross said the failure involved the primary radio communications system that allows the transit control centre and train operators to speak to each other.

Until the problem was resolved, the TTC used a backup system that prevents transit control staff from contacting individual trains.

Instead, the backup system only allows transit control workers to speak to all trains at once, Ross said.

As a result, trains were put into service later than usual due to safety concerns, Ross said.

Ross said the TTC has not experienced this kind of radio system failure to this degree.

“When these things happen, unfortunately, they seem to happen at rush hour and that’s when people notice them,” Ross told CP24.