NTSB regulators investigate why safety mechanism failed to prevent fiery Caltrain collision

The National Transportation Safety Board spent much of Friday sifting through evidence at the site of Thursday's fiery collision involving a Caltrain commuter train in San Bruno. Investigators are looking into why a key safety mechanism, installed on the train, was unable to prevent its collision with a three car work train, carrying two cranes.

"The answer is, we don’t know yet, but I can guarantee you, we will find out," said Bruce Landsberg, vice chair of-the NTSB.

Investigators say the federally mandated positive train control system was designed to prevent exactly this type of crash.

"We can confirm the positive train control was on and active on the accident train, at the time and day of the accident," said John Manutes, NTSB investigator in charge.

The train was carrying 75 passengers when it struck the work train Thursday about 10:30 a.m. in the middle of maintenance work for an electrification project.  

Thirteen people were injured and five people went to the hospital, but officials said none of the injuries were life-threatening. At least two of those people are Caltrain workers and the driver of the crane was sent to a local burn unit.

"The engineer, from what we know, applied full braking and was blowing the horn, and everything to warn the workers to get out of the way," said Landsberg.

The NTSB says the actions by the Caltrain engineer and a foreman on the work train, who warned others aboard, saved lives.  

"Fortunately they were able to get out of the way, because if anybody had stayed there they would not have survived," said Landsberg.

The train was traveling around sixty miles per hour at the time, below its speed limit, but the impact, still pushed the wreck 500 feet. 

The crash ignited a fire beneath train, which is believed to have erupted from the fuel tank of one of the work train cars.

MORE: Fiery Caltrain collision with construction cranes injures 13, NTSB investigating

NTSB officials say much of the wreck will soon be transported to another facility, and a full investigation could take up to 21 months. They will also be analyzing a data recorder, communication transcripts and video from the train.

On Friday morning, the train was still stalled in its track and the crane was stuck underneath it. 

San Bruno Fire Chief Ari Delay said fighting the flames was challenging.

"We were able to suppress the fires we had a stubborn fire under the train," he said. "Some flammable metals that are really difficult to put out and don’t go out immediately. So that took quite some time to get accomplished." 

Caltrain officials say the equipment was supposed to be cleared off the tracks by the time the train rolled through. 

They say it's the same project crews have done for years so they're not sure what went wrong and plan to cooperate with the NTSB, which has sent six investigators to San Bruno. 

Caltrain says a bus bridge will continue to provide service between South San Francisco and Millbrae on Saturday.