Capacity in west coast ports will stop being an issue once the Panama Canal expansion is completed. Then the capacity problem will be on the east coast, with all the large container ships from Asia coming directly here rather than going to the west coast and trucking or training cross country. Not because it will be faster (it probably won't be), but because it will be cheaper and logistically simpler.
Which brings up the point that, for most goods which are manufactured in East Asia, the primary concern in shipping them to the US is cost, not speed. Even if it is faster, a Bering rail connection won't draw as much traffic as you might think because it will also be more expensive. The Russian part being a different rail gauge will only add to the extra cost.