WASHINGTON (USA Today) - Federal investigators say a 30 mph speed-limit sign was posted along the tracks two miles before a curve where the train traveling at 78 mph derailed, killing three passengers and injuring 62 passengers and crew near Seattle last month.

The National Transportation Safety Board (NTSB) investigators also said that had an automatic-braking system been operational, it would have applied the brakes to slow and stop the train.

Central Puget Sound Regional Transit Authority, known as Sound Transit, which owns the tracks at the DuPont, Wash. accident site, said it had installed the technology but was still testing it when the train derailed. Sound Transit has said it expects to have it in operation by the end of June.

Federal investigators said Thursday they have not interviewed the train's 55-year-old engineer or a 48-year-old conductor who was familiarizing himself with the route in the locomotive because of their injuries. The engineer has worked for Amtrak since 2004 and was promoted to engineer in 2013.

The NTSB hasn't concluded what caused the Dec. 18 crash. Train cars dangled over Interstate 5 after leaving the tracks, and the accident injured eight people on the highway. The damage was estimated at more than $40 million.

Amtrak CEO Richardson Anderson has said the railroad will cooperate fully with the federal investigation, but that he couldn't comment on any details of what caused crash while the investigation is pending.

NTSB could take a year or more to complete its investigation.Determining why the train was going more than twice the speed limit into a curve – and how that might have been prevented – is expected to be a key part of the investigation.

The train — carrying 77 passengers, five Amtrak crew members and one Talgo worker for the car manufacturer — was on the first passenger-service trip along tracks installed to shave about 10 minutes off the four-hour ride between Seattle and Portland.

The tracks were posted for 79 mph in the region approaching the crash site, and the train was traveling about 80 mph near the curve, investigators said.

But in their latest update on the crash Thursday, investigators said a 30 mph sign was posted on the engineer's side of the train, to remind engineers about the upcoming curve. Another 30 mph sign was posted immediately before the curve.

Outward facing cameras recorded the engineer commenting that the train was traveling too fast about 6 seconds before the derailment. Inward-facing cameras on the train showed the engineer braking just before the curve and the train left the tracks at about 78 mph, investigators said. He didn't appear to use the emergency brakes, investigators said.

A passenger injured in one of the cars along the highway, Blaine Wilmotte, 24, who suffered several fractured limbs, filed a lawsuit Thursday in Washington state court.

"Amtrak utterly failed in its duties and responsibilities to the victims on the train as well as those who were in the unfortunate path of the train as it left the tracks and crushed numerous vehicles on Interstate 5," said Michael Krzak, a partner at Clifford Law Offices involved in the case. "The hope is that one day lawsuits like these will raise awareness and make rail travel safer."