The sections through Kazachstan, Russia and Belarus is quite long, and it would require a considerable amount of traffic to justify a standard gauge line.

As long as containers are hauled, transloading at the border can be set up to be fast and efficient. As there are also administrative tasks to be done, there would not be that much of a loss of time. And comparing that with the total travel time (maybe 4 hours compare to 18 days), there is not that much of a delay.