The Defense Department has sidetracked all rail cars carrying hazardous material and weapons that have fallen behind schedule for inspection and maintenance, a spokesman said yesterday.

Although department spokesman Christopher Powers said it is too early to determine the number of lapsed cars, informed sources said more than half of the cars in the military fleet, including tank cars holding dangerous fuels and toxic chemicals, have been found to be out of date for required maintenance and inspections.

"We could have done better in ensuring that these cars are as safe as they possibly could be, and we're trying to fix that now," said Powers of the Military Traffic Management Command (MTMC).

An MTMC order to military transport officers to seek approval before using the cars was sent Feb. 8, three days after The Washington Post reported that tons of toxic substances are transported across the nation every year in aged, poorly maintained tankers. The article quoted a House subcommittee chairman and military sources as warning that the fleet posed the threat of a disastrous accident.

MTMC owns and manages about 1,400 flat cars, which transport heavy military equipment, and 1,185 tank cars, which store toxic chemicals and transport them and flammable jet fuels to dozens of military bases in about half the states.

The Federal Railroad Administration (FRA) has 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 shipments of potent fuel for Titan rockets.

Powers said yesterday that a task force set up in December is determining how many of the cars have fallen behind the MTMC timetable for inspections of various rail car parts and the three-year preventive maintenance exams in which a car is jacked up in a shop for a detailed look to guarantee safety and efficiency.

In the meantime, he said, transport officers across the nation have been instructed to seek approval from MTMC before shipping any of the cars.

"We set up procedures years ago and we have been negligent in not following our procedures," Powers said, stressing, however, that cars that fall behind their inspection schedules are not necessarily unsafe.

"The intent here, he said, "is to ensure that none of the cars that are not up to speed will be used until they are brought up to speed."