The Baikal–Amur Mainline (Russian: Байкало-Амурская магистраль, БАМ, Baikalo-Amurskaya magistral’, BAM) is a 1,520 mm (4 ft 11+27⁄32 in) broad-gauge railway line in Russia. Traversing Eastern Siberia and the Russian Far East, the 4,324 km (2,687 mi)-long BAM runs about 610 to 770 km (380 to 480 miles) north of and parallel to the Trans-Siberian Railway.
The Soviet Union built the BAM as a strategic alternative route to the Trans–Siberian Railway, seen as vulnerable especially along the sections close to the border with China. The BAM’s costs were estimated[by whom?] at $14 billion, and it was built with special, durable tracks since much of it ran over permafrost. Due to the severe terrain, weather, length and cost, Soviet general secretary Leonid Brezhnev described BAM in 1974 as “the construction project of the century”.
If the permafrost layer that supports the BAM railway line were to melt, the railway would collapse and sink into peat bog layers that cannot bear its weight. In 2016 and 2018 there were reports about climate change and damage to buildings and infrastructure as a result of thawing permafrost.
Th most important piece of this is that there is a second line that parallels it runs from Moscow in the west to the city of Vladivostok in the east.
The tunnel is 9 and half miles long and saves 15 minutes of travel time.
I thought that the main route for freight between Russia and China was across Kazakhstan to Ürümqi in Xinjiang.