Sakhalin Media: Japanese ambassador noted Toyohisa Kozuki during a trip to Volgograd noted that the construction of a railway crossing between Sakhalin in Russia and Hokkaido in Japan is being discussed with the Russian Federation. 

Ambassador thinks this is a long term project and the priority should be first a link between Sakhalin and mainland Russia. Russian authorities already work on mainland Russia - Sakhalin crossing. There were a railway bridge and railway tunnel options. Because tunnel takes longer to build (10 versus 6), current thinking is bridge. At Eastern Economic Forum 2021, Russian Transport Minister Vitaly Savelyev said that Russian Railways continues the design of the bridge. Go-nogo decision is on Putin.

https://sakhalinmedia.ru/news/1174366/