That’s several thousand miles of additional journey on your way to Shenzen or wherever? Massive amounts of track, tunnel and bridge that has to be maintained in extreme conditions and across multiple nations at odds with each other?

Or just put it on a boat?

If there was actually demand for faster high-volume shipping between East Asia and North America then logistics companies would just buy faster ships. The technology for that already exists and is far cheaper than building a railroad tunnel across the Bering Strait. Shipyards are capable of building freighters with more hydrodynamic hulls and powerful turbine engines. So far no one is really asking for those, which indicates the market doesn't exist.

How is building a rail system for transporting freight to and from the U.S. that runs through Russian territory a good/feasible idea?

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.

Where is Mr Wilford when you need him?

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.