It’s doable. Tunnels and bridges necessary would not be beyond what we have built elsewhere with current technology.
The weather may be challenging in Winter though.
Even if Russia agreed, Putin or whatever strong-man dictator they have next could suddenly decide it was a bad idea, and to block U.S. freight. Then we have no rail transport AND a grounded air fleet that would take time to get operational at the same output it is today.
Having a railroad bridge with russia is inviting problems
Sorry couldn’t resist the Snowpiercer Reference - where they connect alaska to russia with a long ass bridge.
To be fair it does look like a “technically possible but ruinously expensive” kinda project.
Or just put it on a boat?
The one sticky point might be gauge. We'd want something all agree upon. We'd want to avoid variable track gauge bogies.