Thousands of people returned to their homes following a spectacular railroad explosion and fire, but the blast damage threatened a dangerous power shortage Friday as temperatures remained below zero.

"We've got to minimize power usage right down to the last light bulb," said Leroy Lemach, Helena division engineer for Montana Power Co. "It's that critical."As wind chills dipped to 70 below zero Thursday night, National Guard and Air Force troops relieved local firefighters, monitoring the site overnight and extinguishing the last stubborn fires.

"We're just going to keep pouring water on it all night," said Sgt. Ron Weber, a National Guardsman stationed in Helena. "The cars that were the most threatening are now huge ice cubes."

The wreckage continued to smolder Friday morning, said Paul Spengler, Lewis and Clark County disaster services coordinator. The temperature was 29 degrees below zero.

The bitter cold weather was indirectly responsible for the predawn explosions that rocked Montana's capital Thursday, severing power lines and showering some neighborhoods with shrapnel.

Authorities said the accident occurred when railroad workers tried to switch locomotives on a freight train because the heater in one locomotive had failed. Forty-eight freight cars rolled away during the switch, traveling 12 miles into Helena before derailing and plowing into a propane tank.

The tank exploded, igniting a rail car filled with hydrogen peroxide, which also exploded. But a second hydrogen peroxide car was coated with ice, apparently preventing it from exploding.

Only two minor injuries were reported, which Spengler termed "miraculous," adding, "We are all speculating what a disaster we'd have had if that had happened at the rush hour."

Also in Montana Thursday, 52 Burlington Northern grain cars derailed near Glacier National Park, spilling more than 5,000 tons of corn and blocking tracks in both directions. That derailment forced Amtrak to suspend service indefinitely between Minnesota and the West Coast.

The explosions in Helena scattered debris for a quarter-mile radius, showering a nearby college campus and sending a 1,500-pound coupling unit through a dormitory roof. Windows were blasted out of homes and businesses up to two miles away.

About 2,000 residents were evacuated within a square-mile area around the blast site as the temperature reached 30 below zero. Power was knocked out in parts of Helena for up to six hours.

By Thursday night, the evacuation order had been lifted, although parts of neighborhoods near the blast site remained without power. Lemach said the strain on substations carrying rerouted power was so heavy that further problems could force blackouts throughout the city.

Mayor Russ Ritter, who also is vice president of Carroll College, said damage was estimated at $3 million, with $2 million on the college campus. A cement plant near the blast also suffered extensive damage, authorities said.

Ritter said college officials hoped to have the 700 to 800 students evacuated from Carroll's three dormitories back in their rooms this weekend and to resume classes Monday.

Federal officials were investigating whether the brakes on the runaway cars were properly set, were defective or failed because of the cold. Officials also planned to review tapes of radio conversations between the train's engine and the railyard to see whether there was a warning of the runaway cars.

"Right now all we know is the brakes did not work," Montana Rail Link spokesman Fred Simpson said. "The cars went down a steep grade - 2.2 percent - that's steep in railroad terms."