The National Transportation Safety Board is investigating an Amtrak accident that left two CSX workers dead late Tuesday night. The accident investigation disrupted commuter rail traffic and Amtrak traffic.

WASHINGTON — The two CSX workers fatally struck by Amtrak train just north of Union Station late Tuesday night had exited the stopped freight train to check out a technical issue when they were hit by the passenger train, the National Transportation Safety Board said Wednesday.

The fatal collision led to suspended service, delays and disruptions on both the Amtrak and MARC lines into Wednesday morning. Amtrak restored service shortly before 9 a.m. with trains operating at reduced speeds near the crash site. MARC announced the Penn and Camden lines would operate full service Wednesday afternoon along the entire routes after the earlier disruptions, with some minor delays possible on the Penn line because of refueling.

The NTSB is still investigating what led to the accident.

“We have few definitive facts at this early stage,” NTSB board member Earl Weener said during a news conference Wednesday afternoon.

The CSX train was heading toward Union Station from Baltimore Tuesday night when an automatic alert indicated a problem with one or more of the train’s wheels, Weener said. Dispatch prompted the crew to stop the train and check it out and both the train’s conductor and a conductor being qualified on that route exited the train, he said.

At some point before 11:30 p.m., the two workers crossed over on to an adjacent track that was “active” where they were struck by a southbound Amtrak train that had originated in Boston. A CSX engineer who remained on the freight train was not harmed.

The names of the two workers who were fatally struck have not yet been released by authorities.

NTSB investigators are collecting evidence from the tracks, analyzing camera footage and data recorders, and interviewing witnesses, Weener said.

The accident occurred near New York Avenue and 9th Street in Northeast D.C. where there are four parallel tracks — two owned by CSX and two owned by Amtrak.

The stopped CSX train and the seven-car Amtrak train were on adjacent tracks when the accident occurred, Weener said.

The speed limit in that area is 95 mph but closer to Union Station, it slows to 30 mph.

Weener said he couldn’t say whether the conductor of the Amtrak saw the two workers before the collision.

“We are in the process of downloading cameras so we would be able to see what was visible out of the cab as well as what would be visible inside the cab,” he said.

Amtrak trains did not operate during the Wednesday morning rush hour between D.C. and Philadelphia because the tracks were closed as authorities investigated. Service was restored shortly before 9 a.m. at a reduced speed of 10 mph.

Earlier Wednesday, commuters who use MARC faced disruptions and extensive delays on the Penn and Camden lines Wednesday morning.

WTOP’s Neal Augenstein and Colleen Kelleher contributed to this report.