Kim, who took a Air China Boeing 747 jet to Singapore in 2018, could be taking a more scenic route to meet with Trump on Feb. 27-28, Japan's Kyodo News reported Monday.
Quoting a diplomatic source, the news service reported Kim Chang Son, head of Pyongyang's State Affairs Commission, was at the Vietnamese city of Lang Son, at the Vietnam-China border, on Sunday, to visit the train station and check road security in the area.
Recent activity has sources speculating whether Kim will ultimately travel by train. Kim's biological grandfather, North Korea founder Kim Il Sung, traveled as far as the southern Chinese city of Guangzhou in 1958, according to South Korean television network MBC.
Kim Jong Un has previously traveled to China on his personal armored train. Traveling by train will take about three days, and could make matters more complicated, particularly for Chinese authorities who must work with the North Koreans to ensure a safe trip for the North Korean leader.
A source in Hanoi told Kyodo Kim is could be uncomfortable about leaving the North for an extended period of time, but may opt for the "safety" of train travel.
Japan's Fuji TV confirmed Monday "dozens" of North Korean officials were seen visiting a train station at the Vietnam-China border.
The men were seen with people who appeared to be officials of the Vietnamese government, according to Fuji TV.
The total distance Kim would cover is about 2,500 miles.
Yang Moo-jin, a South Korean analyst at the University of North Korean Studies in Seoul, told MBC it is still "highly likely" Kim will travel by plane.
Train travel cannot be ruled out, however. Yang said Kim's land-route option would "strengthen" the "blood alliance" between Pyongyang and Beijing.