According to the Russian Railways website:
The Baikal-Amur Main Line (BAM) was begun in the late 1970s to supplement the Trans-Siberian and provide Russia with a second rail connection from Siberia to the Asia-Pacific region.
BAM’s construction represents an engineering triumph. Stretching nearly 2,000 miles (3,220km) from Lake Baikal in Siberia to Khabarovski Krai on Russia’s Pacific coast, BAM negotiates 7 mountain ranges, 11 alpine rivers and areas of high seismic activity. And for almost half of its length, it runs through the permafrost, where winter temperatures can plummet to −76° F (−60° C).
Building BAM through this difficult terrain required 142 bridges over 100 m long and more than 200 railway stations and sidings, as well eight tunnels, including Severo-Muysky, which at 15.3 kilometres is the longest tunnel in Russia and the fifth longest in the world. Over 60 towns and townships along the route were established.
Russian Railways views the Baikal-Amur Main Line as an investment in the future of Russia which will make possible a substantial increase in the transportation of oil products and exports to the East, a more intensive development and exploitation of raw materials in Russia’s Far Eastern region and an increase in the line’s share in East-West transit traffic.
BAM carried the first trains along its entire length in October 1989, but a much shorter and faster route was opened in December 2003 with the completion of the Severo-Muysky tunnel, which dispenses with the need for trains to switch tracks in order to make a lengthy and time-consuming detour around a mountain. Now that this detour and “double hauling” are no longer necessary, the Company has slashed the time and cost of passenger and freight services and brought BAM up to its full capacity. The Severo-Muysky tunnel has also allowed the Company to greatly relieve the pressure on the Trans-Siberian by switching 6 million tons of freight a year to BAM.
In July 2004, 30 years after construction began on the Baikal-Amur line, Russian Railways announced that it would be investing over 30 billion roubles (around USD1 billion) to develop the Transbaikaliya Railway from Karymskaya station to the Zabaikalsk border station with China.
It is also planned to increase the capacity of the Baikal-Amur line down to Komsomolsk, which entails constructing a second track, electrifying the extension of the prime section and creating repair facilities for a larger locomotive fleet.