The first sign that Kim Jong Un was potentially on his way to meet US President Donald Trump for a second summit was the reported appearance of an armoured train in China.
If that train trundles all the way to Hanoi carrying the North Korean leader, it'll mean a nearly 4,000-kilometre (2,500-mile), 60-hour journey on board for Kim.
He is not long back from his last rail trip. On that occasion, in January, he travelled to Beijing with his entourage in an olive-green train emblazoned with a yellow stripe.
The engine and carriages appeared similar, possibly identical, to the train Kim used the previous year to travel to the Chinese capital for his first overseas visit.
His predecessors, father Kim Jong Il and grandfather Kim Il Sung, also preferred rail for their domestic and overseas travels.
International childhood, domestic rule
Kim Jong Un studied in Switzerland in the 1990s, including at the International School of Berne, along with his brother and sister and is believed to have visited Germany and France during the period.
Unconfirmed South Korean news reports said Jong Un and his brother Jong Chol visited Tokyo Disneyland as children using fake passports to enter Japan in 1991.
Infamously, his eldest brother Jong Nam -- assassinated at Kuala Lumpur's international airport in 2017 in a killing widely blamed on Pyongyang -- tried to do the same in 2001, using a Dominican Republic passport, but was stopped at Japanese immigration.
Kim Jong Un is known to travel by air domestically, and is said to have accompanied his father on a 2011 train trip to China, but it is believed that last year's trip to Beijing was his first journey abroad since ascending to power.
In 2015, the Kremlin announced Kim would be attending ceremonies to mark the 70th anniversary of the Soviet Union's victory over Nazi Germany in World War II, but the visit was cancelled with no reason given.
Fear of flying
Kim's father Kim Jong Il was renowned for his fear of flying, limiting his foreign trips to overland journeys to China and Russia by armoured train.
His 2011 trip to China was a marathon 6,000-kilometre journey, taking in Beijing, Nanjing and Shanghai among other destinations.
Kim Jong Il also took a train to Russia in 2001. According to an account published the following year by Konstantin Pulikovsky -- a Russian official who travelled with him during the three-week trip -- the train was stocked with fresh lobster and cases of Bordeaux and Burgundy red wines from Paris.
He made a second trip to Moscow in 2011, when he met then-president Dmitry Medvedev in the Siberian city of Ulan-Ude.
At the time, residents near the Bureya rail station were told to stay in their houses and not look out of windows as his train arrived.
Of the three Kims, the North's founding father Kim Il Sung was the most frequent overseas traveller.
He secretly visited Moscow in 1949 to meet Soviet leader Joseph Stalin and seek support for his plan to reunify the divided Korean peninsula by force.
The following year, Kim Il Sung's forces invaded the South, triggering the Korean War that pitted Pyongyang's Chinese- and Russian-backed troops against a US-led United Nations alliance.
In 1961, Kim Il Sung returned to Moscow to meet then-General Secretary Nikita Khrushchev and the two countries signed a mutual defence pact.
He was a prominent figure in the Non-Aligned Movement and attended a conference of Asian and African countries in 1965 in Bandung, Indonesia, bringing along his son.
In 1990, he travelled secretly to China, reportedly to discuss warming relations between South Korea and the Soviet Union with Chinese leaders, including Jiang Zemin.
Kim Il Sung's longest train trip was in 1984, a tour of the Soviet Union and other East European countries -- Poland, East Germany, Czechoslovakia, Hungary, Yugoslavia, Bulgaria and Romania.
The carriages Kim Il Sung and Kim Jong Il used for their travels are on display in the Kumsusan mausoleum in Pyongyang, where their bodies lie in state -- with a Macintosh computer on Kim Jong Il's desk.