The TTC has a streetcar mystery on its hands.

The agency said Thursday that despite extensive testing, it’s so far been unable to pinpoint the source of the problem that damaged two dozen streetcars on the busy 501 Queen St. route, and forced it to deploy buses on the line, instead.

The agency is so baffled, it’s contacting other transit agencies to ask for advice.

“It is fair to say, it’s a mystery. Our streetcar crews … are all a bit perplexed as to what caused this,” said TTC spokesperson Stuart Green in an interview.

Usually the TTC operates the 501 route using about 35 of the new-model Bombardier streetcars. Because buses are smaller, it takes about 90 of the vehicles to provide close to the same capacity as streetcars.

The agency said it expects the streetcars won’t be back on Queen until Monday morning.

About 55,000 people ride the 501 on an average weekday, making it one of the busiest surface transit routes in the city, according to statistics from the agency.

While it sorts out the problem, the TTC is advising customers it will have additional service on the nearby 504 King route.

The TTC made the decision Wednesday to pull streetcars from Queen after 25 of the vehicles suffered similar damage to their emergency braking systems.

The affected part is a heavy metal component that sits low in the streetcars’ wheel housing and drops onto the tracks to supplement the streetcar’s disc brakes when braking hard.

Green said the component was damaged to varying degrees on the 25 cars. In some cases, it was completely knocked off, and, in others, it was merely scuffed up enough to require repairs. In all cases, the damage was done to brake systems on the left side of the cars, leading the TTC to believe there was a single source of the problem.

“It would take some kind of impact to really do the damage that we’ve seen. It’s consistent with a moving vehicle and an object, a stationary object, coming into contact,” Green said.

To try to locate the source of the issue, TTC crews affixed a small camera to the underside of a streetcar and overnight Wednesday ran it up and down the entire length of the 501 Queen route, from Neville Park Loop to Long Branch Loop, a distance of about 23 kilometres.

The crews painted the affected part a bright colour in order to make any damage apparent, but couldn’t locate any problem on the test car.

The TTC sent three more streetcars out Thursday to run the length of the 501 route in another attempt to identify the source of the trouble.

Green said the TTC had moved on to inspecting “diversionary” tracks connected to the main 501 Queen route in places such as Shaw Street and York Street. The tracks are used to redirect streetcars from their regular routing on Queen.

Green said it’s possible whatever caused the problem “has sort of worked itself out,” and the TTC has determined Queen “is more than likely okay.” But he stressed the agency wouldn’t return streetcars to the 501 route until it was confident the problem has been resolved.

“We will not put service out unless it’s absolutely safe to do so,” Green said.

The streetcars have many braking systems, so the TTC isn’t worried the damage will make the vehicles unable to stop, but a dislodged part could cause a derailment, Green explained.

It will take some time to fix all 25 damaged vehicles. The TTC doesn’t have that many of the affected parts on hand, and is attempting to find a source for them. Green said Bombardier is assisting with the repairs.

Initially, he said Thursday the TTC hoped to return streetcars to Queen as early as Friday, but, in a statement later in the afternoon, the agency announced they may not be back until Monday morning “or until such time as the track is declared safe for operations and we make significant progress on repairs.”

According to the statement, TTC CEO Rick Leary directed staff to reach out to other rail operators in North America “to ask if they have seen this kind of issue before.” The agency is also speaking with consultancies that have rail and engineering experience.

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The repairs and track inspections are requiring TTC employees to work overtime, but Green couldn’t immediately say what the full cost of addressing the damage will be.

He said the TTC doesn’t believe the problem is related to a manufacturing defect because only cars on the Queen route have been affected.

“This is more than likely a case of something in the track system. However, we have to, out of due diligence, look at all possibilities,” he said.

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