I think you are right. Hostage released?
Report today that Kim has always been too endangered, as was his father before him, to ever consider leaving the country. His absence from country if known by his military could/would enable an instantaneous coup by the generals of the military.
I had to wonder how Kim could pull this train trip off, outside the military eyes on him. Also, wondered if his in-country military enemies are relieved to hear of tensions relaxing if only he makes a deal with Trump?