The passenger death toll in the derailment of the Southern Crescent yesterday in the hills of western Virginia is the largest in a railroad accident in the United States in six years.

The six fatalities are the most since 45 persons were killed in the collision of two commuter trains in Chicago in 1972.

It also is the largest number of passengers to die in a derailment since June 10, 1971, when an Illinois Central train traveling at about 90 miles an hour left the tracks near Salem, Ill., causing 11 deaths.

Yesterday's incident came during a year in which at least 17 persons died as a result of two separate freight train derailments and new attention was focused on rail safety.

Eight persons were killed Feb. 26 in Youngstown, Fla., in a derailment in which a tank car ruptured and spewed deadly chlorine gas across a highway. Five days earlier, nine persons were killed in Waverly, Tenn., in the explosion of a derailed tank car that carried propane.

Federal officials say derailments have been increasing as many railroads face financial problems and allow track and roadbeds to deteriorate.

However, officials interviewed last night described Southern Railway, which operates the Crescent, as a profitable line and praised its safety record.

In addition, a Federal Railroad Administration spokesman said that main-line routes, which are most profitable, generally receive the best maintenance.

On such routes, he added, "it is not totally inaccurate to say" that accidents are more likely to be caused by defective equipment than by trackage.

One official estimated the total number of derailments in the nation last year at about 3,000 but said only a few involved passenger trains.

On Jan. 27, 1970, a Richmond, Fred ericksburg and Potomac passenger train derailed just south of Alexandria, killing three passengers and injuring 41. The probable cause was linked to inadequate track maintenance.

On June 29, 1969, one person was killed and 147 were injured in the derailment of a Seaboard Coast Line train near Glenn Dale in Prince George's County. It was blamed on buckling of welded rails.

The Crescent was involved in a previous derailment on Dec. 6, 1976, when it struck a broken rail near Culpeper, Va., about 70 miles southwest of Washington. Twenty persons were injured, two of whom were hospitalized.