Kommersant publishes the preliminary information over Russian Railways's expansion plans for the 3rd stage BAM. The line is important for Russian coal exports to China. According to Kommersant, there exists 2 options. The cheap one costs 900 billion rubles and the expensive one costs 2,55 trillion rubles.

The 2,55 trillon rubles alternative includes a new Kuznetsovsky tunnel (50 billion rubles), in addition to other expensive infrastructure. Notice that none of alternatives includes the second Severomuisky tunnel (cost 200 billion rubles).

Kommersant gives good arguments for options with details:

https://www.kommersant.ru/amp/4996349