Russia and NK share a short border. I am not sure if the terrain is suitable for moving larger numbers of troops, but they do share a border.
The border is a major river with just a single bridge connecting NK with Russia. But no military force is gonna go that route without approval from China.
I should have looked at a map. Sorry.
If they share a border, then there is a rail link through there.
If there is a rail link, they CAN invade.