The Soviet Union is scrambling to turn around its flagging coal output, and nowhere is that effort more apparent than in this new Siberian coal town 4,500 miles from Moscow. And no example better illustrates the pitfalls Moscow faces in its drive to expand production rapidly.

Output is falling short of targets, even though miners here are the highest paid in the country. More than a third of the heavy-duty foreign-made dump trucks sit idle because of a lack of spare parts. And production costs in this bowl-shaped open pit 1,150 feet deep -- projected by Soviet officials to be a fraction of the cost at shaft mines elsewhere -- exceed expectations.

Effort is not the problem.

Around the clock, even in temperatures of 45 degrees below zero -- which are common in these parts -- coal-excavating trucks bite through rock and frozen earth into the 430 million tons of low-sulphur steam- and coking-coal deposits that make this one of the most promising coalfields in the country.

Neryungri's annual capacity of 13 million tons places it in the second rung of the Soviet Union's major coalfields, after such giants as the Kansk Achinsk and Iuznetsk basins.

The 13-year-old field is part of an all-out effort, spelled out in the draft 1986-1990 economic plan, to steadily boost coal production by tapping new open-pit mines, rather than delving deeper into the traditional underground pits.

The goal is to hike national production from an estimated 1985 level of 720 million tons to an annual rate of 800 million tons by the beginning of the decade.

Last year's national output fell below the projected 770 million tons targeted in the 1980-1985 five-year plan.

The Soviet Union's falling oil production has led officials to stress development of other energy resoures, such as coal and nuclear power.

That involves gradually shifting the mining strongholds from traditional coal-producing areas in the Ukraine or Moscow regions to the open-pit mines concentrated in different parts of Siberia, Soviet energy experts say.

But judging by the Neryungri experience, the inclement weather, desolate conditions and high production costs in Siberia pose some barriers to the Soviet central planner's targets.

Permafrost, or permanently frozen ground, in the area requires the removal of seven times as much earth by explosives as in nonpermafrost areas, according to Yuri Zakharov, chief engineer of the mines. In addition, the 3,500 miners in the pit and 3,000 at the nearby truck depot are compensated for the wear of brutal eight-month-long winters with double the salary they would earn in less-inclement areas. And they must bring in five times the special equipment that open-pit mines in other regions might require, Zakharov said.

Two local officials said that the resulting bloated price of 19 rubles a ton, while still cheaper than the coal dragged from 1,000 feet in shaft mines in the Donets Basin, is more than they had projected.

Although they declined to say what the originally projected per-ton production price was, deputy coal minister Grigori Nuzhdikhin said in a recent national televised discussion that open-pit Soviet coal mining was 10 times as productive and five times cheaper than shaft mining.

Boris Kholkachov, deputy director of the mine, said harsh weather brings about a production shutdown of only three to five days a year. But Neryungri's 1985 output fell well below the 13-million-ton target.

One big problem is machine breakdowns, local officials said. Of the 83 American-designed dump trucks of 180 tons, 34 are regularly out of commission because of a shortage of spare parts, Kholkachov said. And several of the ten 36-ton excavating scoops suffer longstanding operating difficulties in the subzero weather.

The excavators, designed by the Marion Power Shovel Division of Dresser Industries, were built under license by Sumitomo in Japan.

The experience with the machinery has left local officials bitter. Alexei Bisirlik, one of the foremen in the coal-truck depot, told a group of journalists: "There will be no more imported equipment."

After initial setbacks, the rail transport of Neryungri coal to the Pacific ports, and export to Japan, are now under way.

In 1974, the Soviet Union mortgaged half of the 290 million tons of coking-coal deposits to Japan for $450 million in credits to develop the mine and the infrastructure of the nearby town of 60,000.

Both Japan and Soviet officials have expressed satisfaction with the venture.

Two years ago, North Korea also contracted to receive 600,000 tons a year of the Neryungri coal.