The National Transportation Safety Board said yesterday the fact that an Illinois Central Gulf freight train was being operated by the untrained companion of an engineer who had been drinking was among the factors causing a massive derailment last Sept. 28 in Livingston, La.

The derailment of 43 cars forced the evacuation of about 3,000 Livingston residents for more than two weeks while a spectacular chemical fire fumed and burned. The railroad does not expect to reopen the line until this fall and has removed more than 50,000 cubic yards of earth that were contaminated when cars split open.

The board said the derailment started when a damaged air hose coupler between a locomotive unit and the first freight car disengaged. The brakes on the train automatically locked.

An experienced engineer, the board said, immediately would have unlocked the locomotive brakes to stretch the stopping distance and reduce the likelihood of a derailment. However, Janet Byrd, a railroad clerk operating the train at the time, applied the locomotive brakes.

Empty cars were located near the front of the train with loaded, heavier cars behind them, increasing the likelihood of derailment, the board said.

Another contributing factor, the board said, was "the impairment of the engineer's faculties by alcohol and his abandonment of the locomotive controls to an unauthorized and unqualified person . . . . " Byrd and engineer E.P. Robertson were fired by the railroad.

Illinois Central Gulf said yesterday that "no human factor put 43 freight cars on the ground." The ICG has contended that the accident was the result of a broken pin connecting the body of one of the gondola cars with a set of wheels.