By John Ramos

SAN FRANCISCO (KPIX) — A door flew off an San Francisco train while it was traveling underground with passengers on board.

At About 7:30 p.m. on Tuesday, something happened to the door of a Muni train car.

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A small metal stop at the end of a runner broke off and the bottom of the door came off its track as the car entered an underground tunnel.

SFMTA Director of Transit John Haley said, “The air and the wind in the tunnel made the movement of the door a little more pronounced.”

As the train traveled between Castro station and Church station the loose door suddenly rammed a signal post and was ripped from the train.

Muni rider James Chang posted pictures of the door-less car on Twitter. Luckily, no one was sitting in the seat next to the door.

“It is an aberration. It’s unusual. We’re very fortunate that no one was injured,” Haley said.

But is it unexpected?

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Muni says now the doors have all been checked and the cars are safe but many of them have been running day and night for 21 years. That’s 21 years worth of vibration that can cause metal fatigue.

San Francisco resident and Muni rider Brent Silver said, “By and large, it’s pretty safe but I know that some of the cars may need updates from time to time.”

Help is on the way, but it may take a while.

Three new cars — built in Sacramento — arrived in January with 21 more to follow in the next two years.

But the first order of cars can only expand the fleet because the federal government will not pay to replace trains that are less than 25 years old.

Muni hopes it can convince the feds that some of its cars need to be put out to pasture.

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“They have served the agency well, the riders well. They deserve a graceful retirement and some of them as soon as possible,” said Haley.