I don’t think Kim Jong Un likes to air travel very much (or at all), and as he doesn’t go places that aren’t connected through land mass, he uses an armoured(!) train instead. He is travelling to meet Putin as I type this.
It is rumoured to include at least 20 bulletproof cars, making it heavier than average trains and unable to go beyond 59 km/h (37mph). His journey to Vladivostok is expected to take an entire day.
I wondered why was a train picked as the preferred travel method (it was such curiosity what brought me here to type this note), and Wikipedia provided a suitable, logical explanation.