A steam-powered train carrying 1,000 railroad employes and relatives on an excursion derailed yesterday afternoon in Virginia's Great Dismal Swamp, injuring more than 150 persons, at least four of them critically.

Fourteen of the train's 23 passenger coaches left the rails and two of them turned over at the eastern edge of Suffolk, Va., on an isolated stretch of track that runs between dense stands of pine and red maple several miles from any paved roads.

Rescue workers were called to the scene from throughout the Tidewater area and from North Carolina as well, but access to the site was difficult, and some victims remained trapped in wreckage for as much as an hour and a half, according to police in nearby Chesapeake, Va.

The train carried employes of the Norfolk & Western Railway and Norfolk Southern Corp., its parent company.

Robert Claytor, chairman and chief executive officer of Norfolk Southern, was at the throttle of the steam engine, known as No. 611, when the accident occurred, a company spokesman said.

Claytor, 64, "is a fully qualified engineer," said the spokesman, Robert C. Fort.

The seven persons believed most seriously injured, including a 10-year-old boy, were taken to the regional trauma center at the Norfolk General Hospital. Four women were listed in critical condition and two men in serious condition at Norfolk General last night.

The 10-year-old boy later was transferred to Children's Hospital of the King's Daughters in Norfolk, where he was listed in serious condition.

Ten other persons were admitted last night to other hospitals in the Tidewater area. The remaining injured passengers were released after treatment for lacerations, bruises and fractures. All were believed from the Tidewater area.

The cause of the 2:10 p.m. accident could not be learned immediately. The Norfolk Southern spokesman said the derailment would be investigated, and that results might be known in a day or two.

According to Fort, the section of track involved forms part of one of the N&W's main lines. Freight trains regularly use it to carry coal from Roanoke and western Virginia to port facilities in the Norfolk area.

The derailment occurred on what Fort described as high-quality, level, welded-rail track that is "some of the best" in the United States. He said he knew of no curves near the accident site.

The speed of the train, which had left Norfolk about 40 minutes before the accident, could not be learned last night.

Fort said the train was carrying employes on an annual outing and was scheduled to travel to Petersburg and then return to Norfolk later yesterday.

"All of a sudden she just started rolling," one passenger told the Associated Press after the accident.

According to Fort, the steam engine and its tender and the first six passenger coaches remained on the tracks. He said the next 14 cars left the track, with the last two of them turning over. Three cars that followed at the end of the train also remained on the track.

Persons who reached the site later said one or two of the coaches apparently had toppled into a ditch that ran alongside the double track line through Great Dismal Swamp, a 106,000-acre wildlife refuge.

Fort, the railroad spokesman, said the coaches belong to the company although it no longer provides regular passenger service. He said many of the cars were 30 to 50 years old, with some older. The steam engine was built 36 years ago by the N&W and removed from service in 1959. It was refurbished in 1982 and made available by the railroad for excursions.