Nearly 200 commuters were forced to evacuate a train and trudge through a smoky tunnel after a small fire shut down a stretch of Toronto’s busy crosstown line for several hours Monday morning.

The blaze broke out on the tracks near the platform at Dundas West station around 7 a.m., prompting the evacuation of that station and a neighbouring one, the city’s fire department and the TTC said.

A TTC spokesman said the fire started when a wooden cover board contacted the third rail.

“What we don’t know yet is what caused the spark,” said Stuart Green, who noted that early indications point to the line’s electrical system, which is 50 years old.

The commission cut power to the area, which left one train stuck in the tunnel just outside the station, he said. The TTC decided to evacuate the train through the back — away from the fire.

A passenger near the front opened a side door, he said, which caused further stress because the train filled with smoke.

“It complicated an already stressful situation for passengers,” Green said.

Videos posted on social media show passengers walking single-file through the tunnel with the help of subway workers. No one was injured, he said.

Service on the line resumed around 10 a.m.

Green said there are two simple pieces of advice to follow if passengers find themselves in the same situation: stay calm and listen to TTC staff.

“It’s easy to say and more difficult to do in the moment, but we have practised evacuations like this many times,” he said.

The TTC is currently undergoing an infrastructure audit on that line, he said. In early November, old cabling on the same line had corroded and shorted out the power, causing a loss of service.

He said it’s unclear if they are demanding too much power of the system by running upwards of 45 trains on that line during rush hour. But that is another avenue that is being investigated.

Green said crews will go in to examine the area when trains aren’t running early Tuesday morning to try to figure out what went wrong.

Toronto Fire district chief Stephan Powell said the fire was extinguished quickly, using a combination of carbon dioxide and dry chemical.

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The incident contributed to a chaotic Monday morning commute, leaving some stations packed and many riders venting their frustrations on social media.

“It’s a mess down here!” Jason Lascelle wrote on Twitter. “Find another way to your destination.”

TTC spokesperson Hayley Waldman said there were several emergency alarms activated on board other trains, which made the commute worse.

“It definitely causes a bit of backlog — one small issue is compounded by a second small issue, on top of the fire situation,” Waldman said.

Speaking generally about mid-tunnel evacuations, Waldman said “it’s something that’s happened before but it’s not something that happens with any regularity.”