An Amtrak employee waits for the arrival of passengers to board a train in Miami. (Joe Raedle/Getty Images)

Two CSX workers who were fatally struck Tuesday night by an Amtrak passenger train near Union Station had gotten off their freight train to inspect a problem, the National Transportation Safety Board said Wednesday.

Amtrak train No. 175, which had 121 passengers on board, hit the two men — a conductor and a conductor trainee — as the pair tried to identify what triggered an alarm that something was wrong with the wheels, causing the train to stop, NTSB officials said.

The crash caused major delays along parts of the busy Northeast Corridor on Wednesday morning, with Amtrak service between Washington and Philadelphia suspended for nearly 10 hours. Service resumed midmorning, but trains were running at reduced speeds while investigators were at the scene. MARC commuter rail lines in and out of Washington also were suspended for a time.

Part of New York Avenue, a busy commuter thoroughfare, was closed for the morning rush hour, and Maryland transportation officials warned that there could be more delays for the evening commute.

NTSB officials said an investigation is underway to determine what happened Tuesday night. NTSB member Earl F. Weener said it was unknown Wednesday what communication, if any, there was between CSX and Amtrak before the crash.

The CSX freight train, coming from Baltimore, was approaching Union Station when a detector that looks for abnormalities was triggered, officials said.

“The dispatch was notified that there was probably something wrong with one of the wheels, and the train came to a stop for the crew members to take a look and identify the problem before continuing,” Weener said. He said the process is a routine part of train operations.

The engineer remained on the locomotive, he said. But the conductor and a conductor being qualified on the route stepped outside.

At some point, they crossed onto an “active track on which Amtrak” was running its passenger trains, Weener said. The Amtrak train, headed to Union Station, hit and killed the two CSX crew members.

It was unknown Wednesday whether the Amtrak’s engineer saw the men and tried to stop, Weener said.

Train 175 was coming from Boston and New York. The Northeast Corridor is the busiest in North America, with approximately 2,200 Amtrak, commuter and freight trains operating over some portion of the Washington-Boston route each day, according to Amtrak.

The crash happened in an area where four sets of tracks come together — two of which are Amtrak’s and two of which belong to CSX. Weener said the CSX train was crossing from one track to another and the Amtrak train was on an adjacent track. The crash happened near New York Avenue and Ninth Street NE, about a mile and a half northeast of Union Station near Gallaudet University, according to fire and rescue officials.

“Our mission here is to understand not only what happened, but why it happened so that similar events can be prevented in the future,” Weener said.

The identities of the workers who were killed have not been released out of respect for the privacy of their families, CSX spokesman Rob Doolittle said.

The CSX train was traveling from Baltimore to Chicago, carrying intermodal containers with paper products, machine parts, clothing and grain, Doolittle said. He said the train consisted of two locomotives, 49 loaded rail cars and nine empty cars.

No one aboard the Amtrak train was injured, and the passengers eventually were transferred to another train.

The Federal Railroad Administration is assisting in the investigation and is launching its own. Officials with the NTSB said they will look at several things, including the condition of the tracks, cameras in the area and other recordings, along with communications between CSX and Amtrak.

An Amtrak spokeswoman said normal service had resumed by Wednesday afternoon. One track was still closed for the investigation, but trains were operating at normal speeds, spokeswoman Kimberly Woods said. Service is expected to operate on a normal schedule Thursday, she said.

At Union Station, the lines to board trains were long by noon, and many frustrated passengers complained to Amtrak workers about the delays. Some didn’t know the cause of the delays.

“Got to roll with the punches,” Lori Finkel said as she stood in line to board a train to Philadelphia around 12:30 p.m. She had waited awhile to board, she said, and was getting impatient —until she found out what caused the delay.

“That’s terrible,” Finkel said.

Peter Hermann and Lori Aratani contributed to this report.