Not to mention that most of the railway trackage that would have to be built, in both the Russian and the American sides, would be mostly going through permafrost. Permafrost is not the most stable foundation for a railway bed - when the Chinese built the Qinghai-Tibet railway they had to include passive cooling with ammonia refrigerant to keep the soil at a stable temperature, and avoid warping of the tracks. And even then, they are running the risks of having to reconstruct the permafrost section of the track due to unanticipated global warming effects. The chinese only had a short 500km section with which they had to contend with permafrost. The Russian-American railroad would have to contend with thousands of kilometers of permafrost.
And if there were really that much of a business case for a US to China railway connection, the same case could be argued for a China to Europe railway connection,which already exists. Yet despite being a more direct route to Europe than an ocean route, the existing Eurasian Land Bridge only carries 1% of the China-Europe trade. The vastly more expensive US to China connection would be an even more dubious business case.