SAN BRUNO (CBS SF) — Federal investigators began the painstaking process Friday of unraveling the events leading up to a fiery crash between a Caltrain engine and construction equipment that injured 13 people and has shut down rail service between the South San Francisco and Millbrae stations.

Caltrain officials said the equipment was taking part in the system’s ongoing electrification project, but it should not have been on the tracks as the commuter train with 75 passengers aboard approached.

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“All the equipment should have been cleared of the tracks when trains come through,” said Caltrain spokesperson Dan Lieberman. “We’ve been doing construction on the tracks throughout electrification. That’s under investigation.”

Neighbors who live near the crash site told KPIX 5 it was not uncommon to see workers near the tracks.

“These guys are working on this whole line every day,” said area resident Larry Valencia. “I know they’re super safety conscious and all of that, but why was that train flying on that track where the truck was?”

That will be among the many questions federal investigators will be trying to answer.

“There are watchmen on site that are communicating with dispatch, but ultimately, it’s not clear where the breakdown took place,” Lieberman said. “This is not exactly a happy day, but we’re glad it is not worse than it is.”

Caltrain officials said a total 13 people were injured in the collision, but none of the injuries appeared to be life threatening. San Bruno Fire officials said that five of those injured, including two Caltrain employees, individuals were transported to area hospitals.

“Our deepest thoughts and sympathies are with all those who were injured today,” said Caltrain’s acting executive director Michelle Bouchard in a statement issued by the agency.

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Early Friday morning, commuters riding on the rail line between San Jose and San Francisco were forced to use a bus bridge between the South San Francisco and Millbrae stations. SamTrans and BART were also providing mutual aid.

The collision near Scott Street in San Bruno happened around 10:38 a.m. when the train slammed into two pieces of equipment on the track.

The force of the impact ripped the equipment apart, with the burning cab remaining in front of the engine and another large piece of wreckage coming to rest several cars back. Burning gasoline could be seen under several cars.

Passengers told KPIX about the terrifying moments that followed.

“It was pretty chaotic. There was maybe 15 seconds where nobody knew what to do,” Jesse Miller told KPIX. “Some people started opening up the emergency exits in the windows. Some people actually jumped out of the windows.”

Miller posted an image on Twitter of the fireball moments after the crash.

“I saw one worker who was taken away in an ambulance. He looked like he got some cuts and serious bruises,” said Caltrain passenger Elizabeth Tom. “I saw another passenger, he had a gash on his eye.”

While investigators piece together the crash scene, rail traffic through the stretch of track has been shutdown.

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The bus bridge will be in place until at least 4 p.m. Friday. For the rest of the system, Caltrain will continue to operate under its regular weekday schedule. Passengers traveling between the South San Francisco and Millbrae train stations should expect service delays as a result of the bus-bridge. Passengers can also find other public transportation options by visiting 511.org.

Source: sanfrancisco.cbslocal.com.