July 21 (Reuters) - A Boston-area commuter jumped off a bridge into a river on Thursday morning after a fire on board a transit train forced passengers to escape through windows and emergency exits on to tracks high above the swirling waters, officials said.

The passenger safely swam to shore and appeared unscathed after her unexpected morning plunge into the Mystic River, just north of Boston, the Somerville, Massachusetts Fire Department told Reuters.

The Orange Line train, operated by the Massachusetts Bay Transit Authority, was traveling south on the rail bridge when the fire broke out, forcing it to a stop. None of the 200 passengers or crew were injured, the MBTA said in a statement.

"This particular location is really an acute vulnerability," MBTA general manager Steve Poftak said at a news briefing. "You're a significant distance from either one of the stations and you're up there on the bridge."

The passenger who jumped off the bridge did not want to be identified. She told CBS News Boston that she felt "safer in the water" than "walking down the tracks" because of the chaotic scene there.

A fellow commuter told CBS that passengers heard a series of explosions. "I thought we were going to be trapped in there and we were going to die," she said.

Video obtained by Boston25 News shows flames and smoking billowing from one car of the train.

The fire started when a metal seal under one of the train cars came loose and made contact with the third rail, which provides electric power to the cars, Poftak said. It is unclear how the seal came loose.

Power was cut soon after the train stopped so that the third rail would not endanger anyone, the MBTA said.

The 42-year-old Orange Line train was serviced on June 23, including an inspection of the seal that detached, Poftak said. MBTA trains are inspected every 12,000 miles, which is every two to three months.

The MBTA has had a number of safety incidents in recent years, including a collision last month.

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