He did come by train. We did have a special forces flight head to pyongyang last week. This makes a lot of sense. Hand over the material to china under US supervision. Russia wants to build a literal new bridge into NK.
Prediction:
China will end up "giving/loaning" NK ~200-300 billion dollars that will serve as fund to support a new NK currency dollar peg. China will get guaranteed access to NK natural resource development. SK, NK and Russia will all take the lead on a 10 trillion dollar over 30 years infrastructure development program in NK.
The big question is unification. I don't think China will support it. No unification is probably the real carrot they are buying.