Ok Bering Strait, so only geopolitically, geologically, and financially insane.

Like I kinda see a world where this is a good idea. If ww2 had sparked a long brotherhood between our nations and with europe a high speed rail across the tundras might be a good lower carbon alternative to flying and the Bering Strait is the most land based path between Eurasia and the Americas. Personally I'm not certain a tunnel is better than a bridge for this, but a few days train ride from cascadia to Moscow or Beijing would have plenty of takers if everyone was chill and could take a high speed train ride from Moscow to Madrid (and major cities in between) and from Beijing to Ho Chi Minh City, Seoul, Bangkok, etc. It would leave flying as really only necessary for Australia and island nations like Japan.

That's however not the world we live in, and Americans aren't clamoring to go to Russia or trade with them