TTC streetcars are running again on Queen Street after they were taken off the route earlier this week due to concerns about damage to the braking systems of 25 low-floor vehicles.

Crews have completed inspections along the entire 501 Queen route, from Neville Park to Long Branch, to allow streetcar service to resume, the TTC said on Saturday.

"We did an extensive investigation over the course of several days," TTC spokesperson Hayley Waldman said on Saturday.

"We ran streetcars over the entire length of the track, over the course of 24 hours, over and over again, just to make sure, if there were any problems, that we would be able to find them. In one case, we attached a camera to the bottom of the streetcar to monitor the track."

Waldman said experts and staff investigated potential "track issues" as well. The TTC was not able to find an exact reason for the braking system damage, but it decided the issue had resolved itself and it was safe for the streetcars to return, she said.

"Service has been running smoothly today," she added.

Originally, the TTC had said the agency had planned to resume streetcar service on Queen Street on Monday. Service on 508 streetcar, a weekday peak service route, has resumed as well.

Streetcar service was suspended on Queen after brake damage was discovered on Wednesday, with about 90 buses brought in during peak times as a replacement. The TTC also scheduled extra streetcars on King and Carlton Streets.

TTC spokesperson Stuart Green said the TTC had several test trains out with crews and consultants, running both ways over the length of the route, to try to determine what had caused the damage.

The damage was done to the supplementary brake system that hangs close to the ground on Toronto's new Bombardier Flexity streetcars. The issue was discovered only on streetcars that travel the Queen route.

On all 25 of them, Green said, the damage was on the left side of the wheels, suggesting that each one hit the same obstruction or piece of problematic track.

According to Green, track work in a few areas most likely resolved the problem and nothing new was found.