An Amtrak train derailment hangs over an overpass along I-5 south of Seattle on Dec 18th, 2017.

Workers had cleared all the wrecked train cars blocking a major West Coast highway by Wednesday morning, two days after the Amtrak train derailed in Washington state while speeding onto a bridge, state transportation officials said.

It took about a day longer than hoped, but crews had carted off the biggest chunk by the afternoon: the 270,000-pound locomotive involved in the Monday morning rush-hour crash in the city of Dupont, which killed three train passengers and sent about 100 people to hospitals.

But the biggest chunk still remained: the 270,000-pound locomotive involved in the Monday morning rush-hour crash in the city of Dupont, which killed three train passengers and sent about 100 people to hospitals.

The affected southbound stretch of Interstate 5 will remain closed indefinitely, the Washington State Department of Transportation has said.

"Even once the locomotive is removed we will need to clear debris," the department said in a statement on Wednesday. The roadway may also need repairs, the agency said.

The train was travelling 80 miles per hour in a 30 mph zone, when it careened off an overpass onto the highway below, according to the National Transportation Safety Board.

NTSB member Bella Dinh-Zarr confirmed during a Tuesday evening press conference that the Amtrak Cascades 501 train, which was carrying 83 people when it derailed Monday, had not been outfitted with "positive train control."

"Positive train control" is an overlay system designed to stop train derailment, collisions and speeding accidents by enforcing speed control.

Congress mandated all major rail lines be outfitted with PCT in 2008, after two trains in Los Angeles collided head-on, NBCNews reported. The original deadline was the end of 2015, but Congress extended implementation until the end of 2-18, due to the up to $22.5 billion price tag and the complication.

"Could that speed control have made a difference? We don't know that for sure," Washington Gov. Jay Inslee said during a conference.

"None of us, including myself, should be making conclusions about the causes of this accident at the moment," he added.

Dinh-Zarr, meanwhile, emphasized the NTSB will not release a cause of crash at least until completion of the on-site investigation. She did, however, detail some findings.

Dinh-Zarr said the train's emergency brake had not been activated by the train's engineer, but was automatically activated during the accident. At the time of the accident, the engineer was not alone in the cab. A conductor familiarizing himself with the territory accompanied the engineer, while the train's conductor was intermingling with passengers.

"Distraction is one of our most wanted on our list of priorities. It is protocol to look at all cell records of all crew members," Dinh-Zarr said.

NTSB and the lead investigator plan to thoroughly interview all crew members once they are medically fit. In the coming days, NTSB will continue to collect information from data recorders, as well as two on-train cameras that have sustained heavy damage.

The on-site investigation could last another seven to 10 days, after which point NTSB will likely still need more time before determining the cause of the crash.

The derailment placed Amtrak, the country's main passenger rail service, under renewed scrutiny following a series of fatal incidents.

At least two of the three people killed in the derailment were transit enthusiasts who wanted to ride the inaugural run along the new route. A medical examiner is still trying to identify the third victim through fingerprints, the Washington State Patrol said on Wednesday.

--Reuters contributed to this report.