Juan Alvarez could face the death penalty
Juan Manuel Alvarez, 25, could face the death penalty as he is charged with murder with "special circumstances".
Police believe Mr Alvarez, who was not injured, left his Jeep on a railway line in a suicide bid before changing his mind at the last moment.
He may appear in court on Thursday, depending on his medical
condition.
Some 200 people were injured, 40 of them badly, in the crash in the LA suburb of Glendale, which involved three trains.
'Unmoved'
Prosecutors filed 10 charges of murder on Wednesday night but an 11th charge was being added after the discovery of another body in the wreckage, LA County District Attorney Steve Cooley said.
Mr Cooley said the case was still being evaluated.
"Special circumstances" allegations can make a defendant eligible for the death penalty under state law.
Reports of the detainee's suicidal intentions left the DA unmoved.
"He's not going to engage my sympathy because he was despondent.
"His despondency doesn't move me," he told The Associated Press.
Stuck in the track
Mr Alvarez got out of his vehicle before the two commuter trains crashed during rush hour on Wednesday morning, one of them then hitting a freight train.
According to a boss of rail company Metrolink, David Solow, the Jeep had become stuck between tracks and could not have been moved even if the driver had tried.
Wreckage and bodies were showered over a quarter of a mile (half a kilometre) of track.
Passengers said the impact sounded, and felt like, a bomb exploding.
Dazed commuters who were unhurt by the derailment were seen helping fellow passengers with more severe injuries escape the wreckage.
City work crews hurriedly piled sandbags in street gutters to block streams of fuel leaking from the train mixed with rain threatening to contaminate storm drains leading out to the ocean.
Eleven charges of murder have been brought against the driver who caused the Glendale train crash, in which 11 people died. 

Juan Manuel Alvarez, 25, could well face the death penalty for his actions. It appears he parked his Jeep Cherokee on the railway line with the intention of committing suicide, but then changed his mind and left the vehicle. Three trains carrying approximately 350 people were involved in the crash, injuring up to 200 passengers.

His arraignment was delayed from Thursday to Friday due to his medical condition. He was charged with 11 counts of murder and was held without bail in a hospital's jail ward after apparently slitting his own wrists and stabbing himself in the chest. Prosecutors have filed 10 counts of murder with so-called special circumstances, and will add the 11th victim to the complaint shortly. Under California law, this makes Alvarez eligible for the death penalty.

The accident was caused as a result of a chain-reaction derailment of three Union Pacific trains. Investigators are continuing to try to determine the exact sequence of events.