Some wondered whether Kim had only travelled through China on his way to Russia - a scenario 'almost ruled out' by one South Korean diplomat, given the fact that a direct rail connection exists between the North Korean capital Pyongyang and Moscow.
Wrong rail gage.
Easy enough to change the undercarriage. I remember it being done at the border between Spain and France when I was a teenager.
-ccm
They do have a way of changing the carriages so that trains can pass from China to Russia. I assume they have the same provisions at the North Korean border with Russia.