HELENA, Mont. — In the pre-dawn of a fog-choked, icy February morning, six locomotives were pulling a 49-car train up a lonely mountain grade toward the spine of the Continental Divide.

With the wind chill, it was 71 degrees below. And when the cab heaters failed in the second and third locomotives--and the lead engine quit altogether--the crew stopped to switch the other three locomotives to the front.

But one crewman, fearing frostbite, jury-rigged the brakes on the rest of the train, rather than walk its half-mile length.

And in his haste, the cars slipped away.

They careened down the mountain into the outskirts of Helena, plowing into a pair of standing locomotives. Fifteen derailed cars piled up like metal sausages next to the local college. Industrial strength hydrogen peroxide began leaking from a punctured tank car.

Dripping onto creosote-soaked railroad ties, the peroxide ignited. The fire melted polyethylene pellets in a car upended atop the tank car, releasing flammable vapors.

Eighteen minutes after the Feb. 2, 1989, crash, two explosions rocked Helena--the second so massive that many residents thought they were under nuclear attack.

Windows and electric power were blown out in much of the city. A rail car axle sailed nearly half a mile, crashing a foot away from where 80-year-old Catherine DeBree was sleeping on the other side of a bedroom wall. More than 3,500 students and townspeople were evacuated in the sub-freezing dark, many in bare feet over broken glass and ice, leaving as much as $28 million in damage behind them.

The punctured tank car that unleashed the disaster was the kind that hauls most of the hazardous material, or hazmat, carried by rail in the United States. With its several variations, the thin-skinned model--known by its government label as the "DOT-111A"--accounts for more than half of the hazmat spills on the nation's railways, according to a computer study by The Times of U.S. Department of Transportation data.

Critics say the railroads have shied from improvements to the basic tank car to keep costs down in the highly competitive transportation business, where rail for years has been losing market share to trucking. The railroads acknowledge the competitive pressure but insist their safety record is improving.