Emulating his late father and grandfather, who took epic train trips when they were leaders, Kim set off on the long journey from Pyongyang, with a military honour guard seeing him off in the North Korean capital.
North Korean leader Kim Jong Un's armoured train chugged across China on Sunday as he headed to his highly anticipated second summit with US President Donald Trump in Vietnam.
Emulating his late father and grandfather, who took epic train trips when they were leaders, Kim set off on the long journey from Pyongyang on Saturday, with a military honour guard seeing him off in the North Korean capital.
His departure from the Pyongyang railway station was confirmed by North Korea's official KCNA news agency, with official photos showing him waving from the train for what could be a 60-hour journey to Vietnam.
The train crossed the border city of Dandong later that day, according to South Korea's Yonhap news agency and the specialist outlet NK News, but its route remained a tightly-held secret.
The train's crossing into China follows days of speculation over Kim's travel plans, as his team gathered in Hanoi ahead of the talks expected next Wednesday and Thursday.
Accompanying the North Korean leader was right-hand man and top general Kim Yong Chol, who met with Trump in the White House last month, along with several other top dignitaries, KCNA said.
Security was tight before the train's arrival in Dandong, with police cordoning off the border bridge area with tape and metal barriers, and leading an AFP journalist out of the area.
A hotel facing the bridge was closed for impromptu renovations on Saturday.
"The train is long and crossed the bridge slower than the tourist train, but it's definitely him, there's a lot of police presence," an unidentified source told NK News.
Windows on the train were blacked out, the source said, with only headlights turned on as it crossed.
The train usually takes 13 hours to reach Beijing, but there were no signs of heightened security around the railway station, indicating that the train likely bypassed the Chinese capital on the nearly 4,000-kilometre (2,500-mile) journey to Vietnam.
Kim has met Chinese President Xi Jinping four times in the past year, briefing his country's sole major ally before and after his historic summits with Trump and South Korean President Moon Jae-in.
The North Korean leader, who last met Xi in January, could be saving their next meeting for his trip back to Pyongyang to debrief the Chinese leader.
Trump and Kim met in June in Singapore, producing a vaguely worded agreement on denuclearisation, but progress has since stalled, with the two sides disagreeing over what the agreement meant.
Observers say tangible progress is needed in Hanoi to avoid the talks being dismissed as a publicity stunt.
Kim travelled to Singapore last year on a plane lent by Beijing.
Several sources said Kim was expected to arrive in Vietnam by train, stopping at the Dong Dang train station near the China border, then driving to Hanoi.
Soldiers were deployed to Dong Dang station and along the road to the capital, according to AFP reporters at the scene.
Vietnam previously announced the unprecedented move of closing that 170-kilometre stretch of road on Tuesday between 6:00 am and 2:00 pm - suggesting Kim could travel along it between those hours.
"Vietnam wants to contribute its part to world peace... at this upcoming summit. Vietnam will ensure maximum security and safety to all the distinguished guests to Vietnam," Vietnamese Prime Minister Xuan Phuc told reporters on Sunday.
As Kim travelled, delegations prepared for the summit, with US National Security Council official Allison Hooker spotted at a Hanoi hotel where members of the North Korean side have been seen in recent days.
Rail travel is a family tradition that was started by Kim's grandfather, the North's founder Kim Il Sung, who travelled to Eastern Europe for his longest train ride in 1984.
His late father, Kim Jong Il, travelled all the way to Moscow by train in 2001.
"It sends a strong message to North Koreans that Kim Jong Un has inherited his grandfather's good qualities, and the Kim Dynasty is stronger than ever," said Koh Yu-hwan, professor at Seoul's Dongguk University.
Jeong Young-tae of the Institute of North Korean Studies in Seoul said the safest way to travel would be to take a plane provided by Beijing.
"But by choosing to travel by their own special train over a Chinese aircraft, Pyongyang may be signalling its willingness to be independent," Jeong said.
The journey from China's frozen northern border to subtropical Vietnam presents a logistical headache and complex security challenges.
Zhao Jian, who studies China's railway system at Beijing Jiaotong University, said one route would take Kim straight down to southern China, before heading west into Guangxi province, which borders Vietnam.
Justin Hastings, associate professor in international relations at the University of Sydney, said that would be "a pretty major operation."