The idea of constructing a railway through Eastern Siberia and the Russian Far East was considered as early as the 19th century. After surveying the vast territory from the town of Ust-Kut to the Mui River, General Staff specialists concluded that it was completely impossible both technically and financially. The issue was revisited in the 1920s, and in 1938 the construction of the Baikal-Amur Mainline, or simply BAM, began. The approaches to the route and its first sections were built by prisoners. In total, more than 150,000 prisoners worked on the construction of BAM. In the late 1930s, the area was surveyed using aerial photography because the impassable taiga made ground reconnaissance impossible. Regular train traffic on the first section of the mainline began in 1958. In 2003, BAM opened Russia's longest tunnel, the Severo-Muysky Tunnel, which is 15.3 km long. In 1974, BAM was declared an All-Union Komsomol shock construction project. Youth from all over the country came to work on this colossal Soviet project. BAM is considered one of the largest routes in the world. It stretches over 4,300 kilometers, with a significant portion of the road running through the permafrost zone. This is the most expensive Soviet project. By the early 1990s, its cost had already reached nearly 18 billion rubles. The route crosses 11 rivers, including the Angara and Lena, as well as seven mountain ranges. The mainline passes through six regions of Russia, with more than four thousand bridges and about three hundred stations built along its route. At its highest point, BAM rises to 1,323 meters above sea level, passing through the Mururinsky Pass.