A subway train outside Boston caught fire on Thursday morning as it crossed a bridge over the Mystic River, prompting an evacuation that led many to climb out of windows and one person to leap into the water below, the authorities said.

The fire in Somerville, north of Boston, was caused when a metal strip similar to aluminum siding came loose from a train car and made contact with the third rail, which had electricity coursing through it, Steve Poftak, general manager of the Massachusetts Bay Transportation Authority, said at a news conference.

Mr. Poftak apologized to 200 passengers who had been on the Orange Line train, which had begun the day in Forest Hills and caught fire between the Wellington and Assembly stations around 6:45 a.m. He called the fire “a frightening incident and not the type of service we want to provide to our customers.”

No one was injured in the episode, and power was shut off to the third rail in less than two minutes after the authorities received a call about it, he said. The woman who jumped into the river declined medical attention, the M.B.T.A. said.

“This particular location is really an acute vulnerability,” Mr. Poftak said. “You’re a significant distance from either one of the stations and you’re up there on the bridge.”

Nick Andreucci, 24, lives in Malden in the greater Boston area and was taking the Orange Line to work at Massachusetts General Hospital, typically a 15-minute trip, when he started hearing thuds that became louder and louder. He looked outside and saw smoke, causing him and others to rush to the back of the car and away from the smoke.

“That’s when chaos happened,” he said in an interview Thursday night. “Someone tried to open the back door of the car. The back door wouldn’t budge. That’s when the same passenger ended up like two-leg kicking a window open. That’s when all of us jumped out.”

The fire was the latest in a series of troubling episodes for the greater Boston area’s transit system, known as the T. Leaders have been calling for a greater investment in the public transit system.

Mayor Michelle Wu of Boston called the latest episode on the Orange Line “more evidence of an aging transit system in crisis.”

It was not immediately clear why the metal strip, which is fastened to the train, slipped off, Mr. Poftak said at the news conference.

He added that the current heat wave “obviously” had an impact on the agency’s recent operations, saying that officials struggled with draw bridges sticking in an upright position on Wednesday. He did not know whether the heat played any role in the train fire.

“I don’t want to engage in guess work,” he said. “It will be something that we look at.”

Passengers climbed out of four large windows that had been opened on the train, and the train’s crew members helped others evacuate. A spokesman for M.B.T.A. said in an email that a “vast majority” of passengers got out through the rear door of the last car on the train, and that it wasn’t immediately clear how the woman who jumped into the river left the train. He deferred questions about the episode to the U.S. Coast Guard, which did not immediately return an email.

Mr. Poftak said that service was restored by 11 a.m. after inspections were complete. The train, which was put in service in 1980, was last inspected on June 23, and the seal that came loose had been inspected then, he said.

Mr. Poftak said that the M.B.T.A. and public transit in general were much safer than most other forms of transportation, but he acknowledged that the system had recently “had a series of incidents.”

In April, a man was dragged to death after his arm was caught in the door of a Red Line train he was exiting, according to the National Transportation Safety Board. Last July, 20 people were injured when two Green Line trains collided, and in January 2021 an M.B.T.A. commuter rail crash with a vehicle killed the car’s driver, according to Bloomberg.

“A broken M.B.T.A. threatens the safety of our community and the future of our city and region,” Ms. Wu, the mayor, said in her statement. “I’ll be reaching out to my colleagues across the region to more aggressively partner with the state on rapid systemwide upgrades. The city of Boston must do more to help the state lead this transformation, and we are ready to prioritize this work.”

Mr. Andreucci, the passenger, said he hoped the episode was a wake-up call for officials to invest more in transit infrastructure. But it will not dissuade him from continuing to use the train system because he has become used to the problems, he said. A few years back, not too far from where the fire occurred on Thursday, he said, his girlfriend’s train filled with smoke and many passengers exited with soot on their face.

“I work tomorrow morning,” he said. “Same time. I’ll be there.”

Sheelagh McNeill contributed research.