(CNN) -- Amtrak service resumed Wednesday morning between Washington DC and Philadelphia after two CSX rail employees were struck and killed by an Amtrak train shortly before midnight near Washington's Union Station, the train service and the railroad company said.

Train 175, which had traveled through New York from Boston, was involved in an "incident" while approaching the station, Amtrak said. No injuries were reported among the crew or the 121 passengers aboard the train.

"Our thoughts are with the families and friends of our employees," CSX spokesman Rob Doolittle said in a statement to CNN.

Amtrak warned of residual delays as trains returned to normal Wednesday, adding that some trains in and out of Washington were running at restricted speeds.

Washington's Metropolitan Police Department is working with Amtrak and CSX to investigate the fatal incident.