A federal investigator testified today that an Illinois Central Gulf railroad office clerk told him she was operating a chemical-laden train that derailed near here Sept. 28 while the two crew members were drinking heavily.

The derailment of 43 cars of the 101-car train in nearby Livingston forced the evacuation of 3,000 people and virtually closed that tiny town for more than two weeks while officials dealt with a spectacular fire spewing potentially lethal fumes. Only one person was injured.

The accident revived an eight-year-old issue for federal transportation safety regulators and the railroad industry: whether the Federal Railroad Administration should impose restrictions on train crew members similar to the Federal Aviation Administration's ban on drinking by pilots in the eight hours preceding a flight.

The railroad administration has declined to impose such a restriction, although administrator Robert Blanchette said in a recent speech that "if something does not happen," the railroad industry is "doomed to federal regulation in this area."

The issue was raised anew in connection with the National Transportation Safety Board hearings here today in which the clerk, Janet Byrd, invoked the First, Fifth, Tenth and Fourteenth Amendments when she was called to testify. The board, surprised by the move, then placed chief investigator John A. Rehor on the stand.

Rehor had interviewed Byrd and she told him, he said, the same things she had told a railroad supervisor: that she had joined the two locomotive crew members in their Baton Rouge motel room, had driven them to a convenience store to buy a bottle of liquor and had watched them drink from it.

Rehor said Byrd told him that later that night, when the crew members were called to operate a train from Baton Rouge to McComb, Miss., she drove them to the roundhouse and was persuaded to join them in the locomotive. Railroad regulations prohibit unauthorized persons from riding in the cab of a train.

Engineer E.P. Robertson, she told Rehor and her supervisor, continued to drink as the train moved from Baton Rouge to Livingston, 30 miles east. "Robertson was so bad off he could no longer sit up in his seat," Rehor said she told him. "It was for this reason she had to take over the controls," Rehor said.

Byrd invoked the Fifth Amendment, according to her attorney, Fred Holthaus, because she had been charged by Louisiana state police with "reckless handling of hazardous materials," a felony added to the state code by the legislature just last summer. Livingston Parish (county) District Attorney Duncan Kemp said in a telephone interview that Byrd, engineer Robertson and brakeman Russell Reeves, the other person assigned to the locomotive cab, have all been charged with reckless handling of hazardous materials. They have had no opportunity to enter a plea, however, because evidence in the case has not been presented to a grand jury. He said Robertson's attorney has temporarily blocked that action in state district court.

In the meantime, Byrd, Robertson and Reeves and two other crew members on the train have been fired by Illinois Central Gulf, according to chief spokesman Robert W. O'Brien. They were fired, O'Brien said in an interview, for "rules violations."

What rules? "We don't get into that," O'Brien said.

The railroad prohibits "the use of intoxicants or narcotics" by employes "subject to duty." O'Brien said both Robertson and Reeves were "subject to duty" while on layover status in the Baton Rouge motel.

It would also be a violation of rules for an employe to be an unauthorized operator of a locomotive, O'Brien said.

Robertson and Reeves could not be reached for comment. Officials for the railroad unions representing them said they would appeal their dismissals.

Other railroad employes and two deputy sheriffs testified today that Robertson and Reeves both seemed normal and in no way inebriated, either when they reported for duty or after the derailment. Both men also declined to testify at today's hearing on the advice of their lawyers.

A bartender and a cocktail waitress at the motel where Robertson and Reeves stayed testified that two men ordered two drinks each and took the second one in a paper cup after the bar closed at midnight. At a railroad hearing about a month later, they identified those men as Robertson and Reeves.

The reason for the derailment is still being studied. Rehor and O'Brien said computer simulations of possible sequences are still being conducted; Rehor said one possible scenario was that an air hose coupling failed and the train braked automatically. That had happened twice at slow speeds while the train was preparing to leave Baton Rouge.

The train, according to board officials, was exceeding 40 mph in a 35 mph zone when it derailed. With brakes locked, the dynamics of a long train might cause a car to jackknife, or ride up over another.

An experienced engineer, Rehor said, might be able to avoid the problem through skillful handling of brake and throttle. Byrd told her supervisor, according to his statement, that when the brakes applied automatically, she also applied the hand brake.

The 16th through the 58th cars derailed; seven were filled with vinyl chloride and 11 with phosphoric acid. Two of the vinyl chloride cars ruptured and exploded like rockets. The fire that burned for days fed off the vinyl chloride and seven other hazardous materials carried by the derailed cars. Another car carrying chlorine stayed on the tracks and was pulled to safety. Railroad officials point out that more than 1 million carloads of hazardous materials moved without a fatality in 1981.