Perhaps Kim is afraid personally, but he is certainly not afraid for the life of his envoy.
Out of curiosity I have searched for flights from VVO to SVO on expedia.com, and for example Aeroflot flight 1703 (BOEING 777-300ER) is a 9 hours 00 minutes non-stop flight, for just $189.08 incl. taxes and fees. I cannot imagine that one could pay that little for a week in a train. Just the food alone will cost more.
Well, last time I flew it took 13,5 hours with two stops for refueling in B-737.
I can expect 9 hours non-stop with IL-96 or B-777 which is still longer than regular trans-Atlantic flight.
BTW, You have probably fond specials for tickets because regular price is at least twice as much.
As for train it probably starts at $150 for ‘platzcart’ class and it takes 6 to 8 days normally.