The Defense Department ships tons of hazardous substances across the United States every year in aged, poorly maintained rail cars, posing the threat of a disastrous accident, according to congressional investigators and military sources.

Federal Railroad Administration (FRA) inspectors have found hundreds of safety defects in the toxic tank cars since 1983, ranging from improperly closed jet fuel cars to faulty wheel assemblies on cross-country transporters of rocket fuel.

The Military Traffic Management Command (MTMC), which directs the fleet, has fallen years behind in its schedule of required inspections on many of its 1,185 cars; often failed to comply with an FRA requirement that tank car and pressure valves be certified by outside experts, and operates at least 100 jet fuel cars that were inspected or repaired at a shop known to have used improper techniques and tools, according to military sources.

Acknowledging occasional lapses in judgment and regulatory compliance, MTMC set up a task force in December to review maintenance of the fleet and took out of service a few of the many tank cars whose inspections were overdue.

MTMC officials say that the tank cars are safe, pointing to their accident-free record. FRA regulators generally agree, noting that few of their safety concerns were serious or uncommon for carriers of hazardous materials.

But military transport sources and the chairman of a House Government Operations subcommittee view the MTMC hazardous material fleet as a disaster waiting to happen.

"The Defense Department adheres to the barest minimum in safety standards, and occasionally even those standards aren't met," said Rep. Cardiss Collins (D-Ill.), whose Government Operations transportation subcommittee reviewed fleet operations at a hearing last month. She warned of an accident with consequences of "Bhopal proportions," a reference to the gas leak that killed 2,800 people in Bhopal, India, in 1984.

A military official who asked not to be identified said tank cars have been allowed to deteriorate because of budgetary constraints at MTMC. "Money is the No. 1 factor and safety is No. 2," he said. "Sooner or later it's going to get somebody killed."

MTMC owns and manages the tank cars and arranges with private railroads to transport them. More than 900 tank cars carry flammable or combustible jet fuel from oil refineries and defense supply depots to dozens of military bases in about half the states.

Another 252 "special use" tank cars store and transport hazardous chemicals used in weapons production. Based at 10 defense facilities or contractor plants, they move less frequently than jet fuel tank cars but are expected to be ready for mobilization in a national emergency and are required to meet FRA and Association of American Railroads (AAR) safety standards, military officials said.

Most of the tank cars have been operating since the early 1950s and are nearing the mandatory retirement age of 40 years. Private industry generally uses newer cars because they are larger and considered more economical.

The older tank cars also lack important safety features, including sealed roller-bearing lubrication units for rail car axles. The axles of most MTMC cars receive lubrication from a container called the "journal box." Oil in the box soaks a spongy pad over which the axle rotates. The box must be carefully maintained to keep oil in the box. If the box is allowed to get too dry, the crushing weight of tons of unlubricated metal rubbing against metal will overheat the box, causing it to burst into flame and eventually melt or crack the axle.

Such "friction-bearing" tank cars are relatively rare in private industry -- the AAR has required that they be removed from service by 1991 -- because they are harder to maintain and less safe than the newer roller-bearing units.

An incident Jan. 15 illustrates the dangers of the old-fashioned lubrication method. The journal box of a 37-year-old tank car carrying volatile jet fuel from Houston to England Air Force Base in Alexandria, La., caught fire about 140 miles from its origin. The crew moved quickly to extinguish the flames, preventing what could have been a serious accident if the overheated axle had cracked and the train had derailed. There were 16 jet fuel tank cars on an adjoining track.

A MTMC investigation revealed that on the three remaining wheels of the damaged car, the journal boxes contained water from recent rains. The water had pitted the axles, preventing the smooth coating of oil necessary to avoid friction.

"It's a logical conclusion that the fourth {axle} was pitted and caused a fire," said one source. "The grease on the wheel just heated to flash point. If it wasn't caught in time, the whole thing could've blown."

The FRA inspects MTMC's chemical tank cars as well as its flat cars. In the five years ending last Nov. 1, inspectors found 647 mechanical or power brake defects, mostly in cars carrying hazardous materials, FRA Executive Director Bill Loftus said. If ignored, the problems could have endangered train wheels and braking systems, he said, noting that repairs were quickly made. Loftus did not know, however, how long the defects existed before the inspections.

Inspectors also found that covers on 338 hazardous material tank cars were not securely fastened, creating the potential for leaks. They also were repaired, but Loftus could not say if there were leaks before the problems were found.

"It's not out of the ordinary from what you see in {commercial} tank shippers," Loftus said of safety problems in MTMC cars, which the FRA calls "exceptions" because they are less serious than violations.

According to MTMC regulations, tank cars must be given checkups every six months to assure compliance with FRA and AAR standards. The preventive maintenance inspection, a more detailed examination in a shop where a car can be jacked up, is required every three years to guarantee safe and efficient operation. But relatively few of the cars have received their preventive maintenance test, eliminating an essential safety check, according to sources.

Critics point to the rail fleet at the Vertac Chemical Co. in Vicksburg, Miss. The company produces nitrogen tetroxide, a component of rocket fuel shipped in a liquid state. Exposed to air, the chemical forms deadly vapors that can cause lung failure. If spilled from a tank car in a railroad accident, according to specialists, the chemical could endanger communities miles downwind.

MTMC maintains 14 cars at Vertac to store nitrogen tetroxide and transport it thousands of miles to a defense contractor outside Sacramento, Calif., that builds and tests Titan rockets. In 1987, the 10,000-gallon cars made 13 cross-country shipments.

Eleven of the nitrogen tet cars were overdue for inspection at the time of the congressional hearing in December. Five tank cars were at least seven years behind schedule and three cars were four years behind.

In early November, a MTMC inspector visiting the Vicksburg yard noted that several cars were overdue for inspection and recommended to MTMC regional headquarters in Bayonne, N.J., that they not be used, according to a source. One of the cars that had just returned from California showed excessive wear of brake shoes, indicating that the brake was sticking, the source said. A sticking brake can heat up, ignite grease and oil on the wheels and cause a dangerous fire, or cause wheels to crack. The car had received its last preventive maintenance test in January 1977.

The inspector passed along the brake problem to Mid South Rail Corp., which transports nitrogen tetroxide cars during the first leg of their journey, the source said.

But the inspector's warning to his MTMC superiors and Mid South went unheeded, the source said. Unrepaired, he said, the car was reloaded at Vertac and transported 60 miles to Monroe, La., the interchange point with Missouri Pacific Railroad. The brake defect was finally spotted and repaired by Missouri Pacific in early December, the source said, a month after it was discovered.

Bill Lucas, MTMC's deputy director of inland traffic, said no one at Bayonne can recall the inspector's recommendation that nitrogen tetroxide cars be taken out of service. He said that a brake shoe had worn down, but did not pose a danger.

Mid South President Edward L. Moyers said he was not aware of the brake shoe problem and had no comment.

But, under congressional pressure, MTMC is conducting preventive maintenance tests on two nitrogen tet cars and plans early inspections on three others. The remaining six tank cars will be sidetracked until they receive thorough exams, Lucas said.

Lucas said he does not know the inspection frequency or condition of the hundreds of other tank cars storing or transporting hazardous materials across the country. But sources said poor maintenance is the norm.

Last November, a MTMC inspector surveyed 100 tank cars in several states storing such hazardous chemicals as white phosphorus and unsymmetrical dimethylhydrazine. Eighty of the cars, a source said, were overdue for FRA, AAR or preventive maintenance inspections.