The head of a federal agency investigating the Florida derailment of a Louisville & Nashville train carrying hazardous chemicals said yesterday that the railroad created a "potentially catastrophic" situation by sending out a train so long and so heavy that it was difficult to control.

The train, pulling 122 cars and weighing 10,600 tons, derailed three miles east of Crestview, Fla., on Sunday. An explosion creating poisonous chemical fumes required the evacuation of 4,500 persons from an 80-square-mile area. Twenty-six of the derailed cars were carrying such chemicals as acetone, chlorine, ammonia and carbon tetracholoride.

James King, chairman of the National Transportation Safety Board, noted that the accident occured in a "swamp." Had it occured in a populated area it would have been "potentially catastrophic," King said, because "they wouldn't have had time to evacuate."

He said the explosion created a fire-ball and a vapor cloud of gasses that would have spread quickly over any highly populated center, adding that carbon tetrachloride when heated gives off a phosgene gas that will "kill."

"Put [this chemical mix] all together in a shake-and-bake bag and you've really got a problem," King said.

King told a House subcommittee the L&N has been involved in 34 accidents since 1976; five of them major. In the accidents 33 persons have died, 356 have been injured and $24 million in property has been lost.

"You take a property that has a very poor accident picture, and it puts out a very long, very heavy train with chemicals on 60 cars . . . You create a problem that could . . . be catastrophic," King said.

King said later in an interview that an engineer would have a difficult time managing that kind of train because of the problems of slack and grades as well as the effect of the weight on the track and equipment. The accident occured at the bottom of a grade which was followed by a double curve, he said.

Other railroads with better safety records do not make up trains that long when they are carrying chemicals. "They generally don't exceed 75 cars," King said.

King told the House Commerce transportation subcommittee, "We feel the L&N management has failed to maintain a level of safety and competence in its employes. In those moves where risks must be taken, they are unfamiliar with the rules. It's a management responsibility and they have shown no discipline, no follow through and no leadership."

King also said in the interview that his agency recommended to the Federal Railroad Administration last year that it limit the size and length of trains carrying hazardous material. They also recommended to the FRA that trains crews have information on board about the material they are carrying.

The National Transportation Safety Board has power to investigate accidents but no enforcement or regulatory power.

Rep. James Florio (D-N.J.) asked James Palmer, Transportation Department administrator for research and special programs, why there are no regulations requiring the shipper to put forms on trains identifying chemicals and how to deal with them. "We're exploring a way to do that," Palmer said.

Florio pointed out Canada already does it. "That's something to start with, not explore. A major problem is that local fire departments don't know what to do with these chemicals," Florio said.

He said the fire department in Crestview did the right thing by backing off and calling federal officials. Others might have tried to fight the fire, which could have caused injuries or deaths. CAPTION: Picture, Ammonia cloud hovers in background as workers repair track near a derailment of chemical tank cars in Florida. AP