Two CSX railroad employees were fatally struck by an Amtrak train after stepping onto tracks outside Union Station in Washington, D.C., Tuesday night.

The victims were a conductor and a trainee aboard a CSX train approaching Union Station. They received an alert that instructed the crew to stop and check part of the train, said a National Transportation Safety Board (NTSB) official. A detector had triggered an abnormality in the CSX train, likely with the wheels.

The two victims were struck by the Amtrak train after getting off their freight train.

"We heard the horn blow, and about three seconds later, we heard a clunking sound. My wife thought she heard something hit the window next to her," passenger Walter Duncan said.

Service on Amtrak is suspended and MARC's Penn Line is running on limited service after two people were killed by a train outside Union Station Tuesday night.

The victims were killed on an active track on which Amtrak was operating passenger trains, near the 1200 block of New York Avenue NE. Emergency personnel were called to the scene around 11:55 p.m. Tuesday.

The victims' names have not been released.

No one was injured aboard Amtrak Train 175, which was traveling to D.C. from Boston and New York. The train had 121 passengers aboard, according to an Amtrak release. There was no information immediately available on the number of crew.

An Amtrak spokesperson previously said the victims appeared to be trespassers, but CSX spokesman Rob Doolittle confirmed Wednesday morning that the victims were CSX employees.

"Our thoughts are with the families and friends of our employees," Doolittle said in a statement.

The victims' CSX train was carrying mixed freight and was traveling from Baltimore. The accident happened in an area where there were two CSX tracks and two Amtrak tracks.

The NTSB official said the agency will assess all conditions on the tracks to see what went wrong. The NTSB will be able to review video footage as part of its investigation.

Amtrak service between Washington, D.C., and Philadelphia was suspended for several hours Wednesday morning due to the incident. Service has been restored, but trains will continue to run at speeds under 10 mph until further notice, the NTSB official said.

MARC commuter rail service into Union Station was suspended on the Penn and Camden lines Wednesday morning, but Wednesday afternoon trains operated at full service. Penn Line riders may encounter delays, MARC said on its website.

Metrorail will honor MARC tickets Wednesday afternoon, MARC said. However, this does not include any charges at Metro parking facilities.



Stay with NBCWashington.com and NBC4 for more on this story.

CORRECTION (June 28, 2017, 11:27 a.m. EST): An earlier version of this story said there were 175 people aboard the train. In fact, there were 121 passengers and an unknown number of crew members on the train.