Compared with North Korea's ageing fleet of passenger aircraft, the bulletproof trains offer a safer and more comfortable space for a large entourage, security guards, food and amenities, and a place to discuss agendas ahead of meetings, experts say.
Since becoming the North Korean leader in late 2011, Kim has used a train to visit China, Vietnam and Russia.
WHAT'S INSIDE THE TRAINS?
It is unclear how many trains North Korean leaders have used over the years, but Ahn Byung-min, a South Korean expert on North Korean transport, has said multiple trains were needed for security reasons.
Ahn said those trains have 10 to 15 carriages each, some of which are used only by the leader, including a bedroom, but others carry security guards and medical staff.
They also usually have space for Kim's office, communications equipment, a restaurant and carriages for two armoured Mercedes, he added.
State media photos on Tuesday showed Kim with senior officials taking a cigarette break next to a green carriage emblazoned with gold-coloured crests and trim, and sitting in a wood-panelled office in front of a large gold crest and flanked by the North Korean flag.
On Kim's desk sat a gold-embossed laptop computer, a bank of telephones, his signature box of cigarettes and bottles with blue or clear liquids. The windows were trimmed with blue-and-gold curtains.
A video released in 2018 by North Korean state TV showed Kim meeting with top Chinese officials in a wide train car ringed with pink couches.
In 2020, state TV footage showed Kim riding a train to visit a typhoon-hit area, offering a glimpse of a carriage decorated with flower-shaped lighting and zebra-printed fabric chairs.
In the 2002 book "Orient Express", Russian official Konstantin Pulikovsky described a three-week journey to Moscow by Kim Jong Il, Kim Jong Un's father and predecessor.
In that train, cases of Bordeaux and Beaujolais wine were flown in from Paris, as were live lobsters, according to the book.
HOW DOES IT CROSS BORDERS?
When Kim Jong Un took the train to Russia, including in 2023 for a summit with President Vladimir Putin, its wheel assemblies had to be reconfigured at a border station because the two countries use different rail gauges, Ahn said.
While there is no such requirement for China, a Chinese locomotive pulls the train once it crosses the border, because a local engineer knows the rail system and signals, said Kim Han-tae, a South Korean former train engineer who has written a book on North Korea's railways.
To travel to previous summits with Xi, Kim's specially equipped string of train carriages was usually hauled by matching green DF11Z locomotives, Chinese-made engines sporting the emblem of the state-owned China Railway Corporation, with at least three different serial registration numbers, according to a review of media images.
Ahn noted the serial numbers were either 0001 or 0002, suggesting China was providing him with engines reserved for the most senior officials.
A DF11Z locomotive with the serial number 0003 drew into the Beijing station on Tuesday, pulling more than 20 carriages with a North Korean national flag and its official emblem in gold.
Attached to the North Korean dark green train were six Chinese carriages with bright yellow double stripes, which Ahn said possibly carried Chinese officials who had greeted Kim when he crossed the border.
When Kim travelled across China to his 2019 summit with U.S. President Donald Trump in Vietnam, his train was pulled by a red-and-yellow locomotive emblazoned with China's national railway logo.
The train can reach speeds up to 80 kph (50 mph) on China's network, compared with a maximum of about 45 kph (28 mph) on North Korea's tracks, Ahn said.
WHO USES THE TRAINS?
North Korea's founding leader, Kim Il Sung, Kim's grandfather, travelled abroad by train regularly during his rule until his death in 1994.
Kim Jong Il relied solely on trains to visit Russia three times, including a 20,000 km (12,400 miles) trip to Moscow in 2001.
He died of a reported heart attack in late 2011 while on one of his trains and the carriage is on display at his mausoleum.
The train has been at the centre of state propaganda around the ruling Kim family's embarking on long train journeys to meet ordinary North Koreans across the country.
In 2022, state television showed Kim Jong Un taking what it termed an "exhaustive train tour" around North Korea to inspect corn crops and promote a "communist utopia".
(This story has been refiled to correct the date in the dateline)
