PHILADELPHIA (Reuters) - A conductor critically injured in last week's deadly train derailment in Philadelphia has sued Amtrak, accusing the publicly funded passenger rail company of negligence, his lawyer said on Tuesday, adding to a string of lawsuits since the crash.

Emilio Fonseca, 33, was taking a restroom break in the first car during his work shift when the passenger train went off the rails, attorney Bruce Nagel told a news conference, saying the train suddenly surged forward and then crashed.

Fonseca, who is married and lives in Kearney, New Jersey, suffered a broken neck, back and both shoulders, and was believed to be the most severely injured Amtrak employee, Nagel said. He is among five people still in critical condition and is expected to remain hospitalised for several weeks.

Eight people died and more than 200 others were injured.

Fonseca is the second employee of the passenger rail line to sue over the May 12 crash, filing the action in state court in Newark, New Jersey, claiming negligence and asking for unspecified damages, his lawyer said.

Other lawsuits have been filed by an Amtrak employee who was riding as a passenger and by four passengers who brought their action in federal court in Philadelphia.

The reason the train accelerated from 113 km per hour to 171 kph in the minute before the crash remains a mystery. Authorities have not ruled out equipment malfunction, human error or other possible reasons for the train gaining speed so rapidly.

"There was a sudden surge and then the wreck occurred,"Nagel said his client told him.

Crawling through the wreckage to a field, the badly wounded Fonseca immediately began warning injured and stunned riders to avoid stepping on downed electrified wires, cautioning them until medics took him away, Nagel said.

"In a lot of ways, Mr. Fonseca is a hero," Nagel said. "He was there with a broken back, broken neck, broken shoulders and he managed to continue to warn passengers to be careful of the live electrical wires that were all around the train wreck."

Train engineer Brandon Bostian, 32, who suffered a concussion, told investigators he has no memory of what occurred just before the crash.

The National Transportation Safety Board and the Philadelphia Police Department are investigating.