The CSX freight train derailment that severed the adjacent Metro Red Line early June 19 was the result of a chain reaction set off when a beam broke in a refrigerator car, a CSX official testified yesterday.

When the beam broke in car number 96 of the 134-car train, the brakes on that car took hold while other cars kept rolling, according to Dan Smirl, manager of CSX's Baltimore division. The result was an accordion-like action that strewed 21 cars up and down the track and into a residential neighborhood just north of Metro's Takoma Park station in the District.

No one was injured, partly because Metro was not operating at the time of the 4:30 a.m. accident. However, Red Line service north of the Fort Totten station was disrupted for three days, and safety experts warned that such an accident involving a rush hour Metro train could cause disaster.

That prompted a hearing yesterday by the Maryland General Assembly's subcommittee on law enforcement and transportation. "It's just a matter of time before a commuter train is in the wrong place at the wrong time," said Del. Peter Franchot (D-Montgomery County), a Takoma Park resident. "This is obviously unacceptable."

The Federal Railroad Administration has not determined the cause of the derailment, but expects to complete its investigation by the end of the year, a spokesman said. FRA investigators said in June that they were focusing on crew handling of the train.

A Maryland state official said at the hearing that poor maintenance may have contributed to the derailment. Rust particles on the refrigerator car beam, or center sill, indicated that it had been slightly cracked before the train wreck, but the defect was not detected in two inspections of the train within 12 hours before the incident, said Dominick W. DiFalco, a state assistant commissioner for railroad safety and health.

DiFalco said the derailment "would seem to coincide with the reduction in maintenance personnel in the last couple of years."

CSX officials contested that conclusion. Smirl said that, while CSX has cut all levels of employment recently, it has not reduced the quality of its maintenance.

CSX records show that its fleet of freight cars and locomotives has shrunk more than the size of its maintenance staff in the last two years. The number of maintenance workers has dropped 13 percent since 1985, but the number of freight cars has dropped 24 percent and the number of locomotives is down 18 percent, a company spokesman said.

"There is no cause and effect" between the maintenance staff cuts and the derailment, said Alvin J. Arnett, a CSX vice president.

The final break in the center beam, which supported a refrigerator car carrying vegetables, may have been caused by the pressure on the freight cars as they were being pushed together and pulled apart while the mile-long train passed over rolling terrain, DiFalco said.

When the beam broke, it ruptured an air hose powering the train's automatic brake system. This activated the brakes in an uneven manner, causing some cars to stop while others kept moving, Smirl said. The train was traveling 45 mph at the time, well within the 55 mph speed limit for that stretch of track, officials have determined.

The Federal Railroad Administration and the National Transportation Safety Board declined to participate in the hearing because of their ongoing investigations.

The safety board recently launched a special investigation to look at the hazards posed to Metro along the corridor it shares with CSX, a spokesman said.

The board originally declined to investigate the derailment because it did not involve a death, a passenger train or damage greater than $500,000.

Metro officials at the hearing said they expect to complete their investigation into the derailment within a year.

Equipment failure is the primary cause of train derailments in Maryland, DiFalco said. However, the number of derailments in Maryland has dropped sharply from 96 in 1980 to 23 in 1986, according to state figures.