Part of a mile-long Conrail freight train derailed yesterday afternoon at a tunnel entrance near Capitol Hill in Southeast Washington, sending dozens of police and firefighters to the scene.

Authorities said about 10 cars had left the rails just outside the entrance to the tunnel, underneath the Southeast Freeway near Second Street and Virginia Avenue SE, while inside the tunnel, several more were jammed against its walls.

No injuries were reported. The cause of the derailment was not immediately known.

Fire officials said that when they arrived they found a liquid dripping from an overturned tank car. That appeared initially to present the greatest hazard.

The tank car, according to Deputy Fire Chief Thomas A. O' Connell, "gave us a little apprehension when we first saw it."

However, firefighters clambering around the overturned car soon saw the printed label "Combustible," and "knew we were in pretty good shape," O'Connell said. He explained that substances labeled "combustible" will burn, but they will not do so readily.

More volatile substances such as gasoline are labeled "Flammable," O'Connell added.

Meanwhile, a call by firefighters to railroad authorities yielded the information that the dripping liquid, with an odor characteristic of woods and forests, was in fact pine oil.

The tank car containing the aromatic oil was one of 125 cars in a freight train pulled by four engines that had set out yesterday afternoon from the giant Potomac Yards in Alexandria on a north- and eastbound journey to Selkirk, N.Y., near Albany.

After crossing the railroad bridge over 14th Street and traversing southwest Washington, the lead cars of the train entered the western end of the tunnel that carries the Conrail tracks under Southeast Washington to the edge of the Anacostia River.

Most of the train apparently had entered the tunnel by about 2:30 p.m. when the derailment occurred, and the cars affected were near the rear of the train.

As the cars swayed, toppled and fell from the rails, at least one of them appeared to nudge against the supports of the Southeast Freeway, which passes over the track near the mouth of the tunnel.

"You could see where the cars had scraped them," O'Connell said. However, he said, there was no sign of structural damage to the freeway.

Railroad repair crews were reported on their way to the scene last night. The wreck tore up part of the track leading into the tunnel. Authorities said it would have no effect on passenger trains, which use different tracks. The effect on freight traffic could not be learned immediately.

After the accident was reported, Assistant Police Chief William Dixon went to the scene to supervise police operations there. About 75 firefighters and 20 pieces of equipment, including three foam trucks, were dispatched.