Thousands of Amtrak passengers traveling between Richmond and Washington suffered delays of more than two hours yesterday as crews worked to clear the wreckage of the 12 rail freight cars that derailed Thursday night near Newington Road in Fairfax County.

Federal Railroad Administration investigators were focusing yesterday on the track and equipment as they looked for possible causes of the 6:20 p.m. accident, said spokesman William Loftus. "It does not appear to be a human factor issue," he said.

The Richmond Fredricksburg & Potomac Railroad, which operated the 75-car freight train, had not determined a cause, but spokesman William Griffin said "initial indications are that the track structure was not involved."

The accident caused no injuries. However, two office buildings and five homes on nearby Cinder Bed Road were evacuated for about two hours until fire officials determined that two derailed tank cars that caused concern were not carrying hazardous materials. The cars contained about 300 pounds of soybean oil, Fairfax fire officials said.

The railroad is investigating reports that the engineer refused for almost 40 minutes to show fire investigators documents listing the cars' contents, Griffin said.

The cars that derailed, numbers 55 through 66, carried lumber, plywood and vegetable oil, he said.

The cars derailed, slamming into seven empty cars sitting on an adjacent track, as the train traveled around a two-degree curve at 50 to 55 miles an hour, a speed within the allowed limit, said Loftus of the railroad administration.

The wreckage blocked two tracks until 4:30 p.m. yesterday, when one was reopened, with traffic restricted to 10 mph through the area, Loftus said.

Passengers on nine Amtrak trains that were scheduled through the area while the tracks were blocked were either bused around the accident or their trains were rerouted, said R. Clifford Black IV, an Amtrak spokesman.

For example, the southbound Virginian, which runs from New York to Richmond, stopped at Washington Union Station Thursday night, and passengers were taken by bus to Quantico, Fredricksburg and Richmond, Black said.

The Silver Meteor, traveling from Florida to New York, was detoured yesterday onto a Southern Railway line through Manassas, he said.