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From the outside, the train that carried North Korea leader Kim Jong Un on this week's surprise visit to China doesn't look exciting.

It's painfully slow-moving, and drab green with yellow piping that runs along the side.

But inside the distinctive train, there's a lot of bling. It was previously used to carry Kim's father on similar trips.

"It had a reputation for being an opulent ride for the leader. It maintains the North Korea leader's lifestyle wherever he goes," said Curtis Melvin, a researcher at the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies. "It is capable of carrying the type of food and entertainment to support the leader as he travels."

For Kim's father and predecessor, Kim Jong Il, that meant live lobsters and wine shipped in from Paris, and four young female singers who were reportedly referred to as the "lady conductors."

State media revealed on Tuesday that Kim Jong Un had met with Chinese President Xi Jinping at the Great Hall of the People in Beijing on an unofficial visit. China is North Korea’s neighbor and most important ally.

It's not clear what the interior of the train might look like these days. Past glimpses of it during Kim Jong Il's regime revealed it to be a party train on the inside and a massively secure form of transportation on the outside.

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It has tinted windows, and according to a 2009 South Korea news report based off of classified information about how Kim Jong Il traveled, is heavily armored. It's also accompanied by many security personnel.

"To defend Kim against attack, two separate trains precede and follow the main entourage, one handling reconnaissance and the other security," South Korean newspaper Chosun Ilbo said of Kim Jong Il's travels. "The advance train handles security checks to see whether the rail tracks are safe. Behind Kim's train is another carrying his bodyguards and other support personnel."

Kim Jong Il's train consisted of 90 carriages, Chosun Ilbo reported, and traveled at an average speed of 60 km per hour — a meager 37.28 miles per hour — while 100 security agents would sweep stations for bombs before the train pulled in.

For Kim Jong Il, who was rumored to have a fear of flying — plus a flair for lavishness — it was a comfortable way of travel.

Russian official Konstantin Pulikovsky, who accompanied Kim Jong Il and his entourage on a three-week journey to Moscow in 2001 and wrote about the experience in a 2002 memoir, "Orient Express: Through Russia with Kim Jong Il," described meals on the train that lasted for hours and included fresh live lobster or other delicacies.

''It was possible to order any dish of Russian, Chinese, Korean, Japanese and French cuisine,'' Pulikovsky wrote.

For entertainment, there were Russian and Korean songs: Kim Jong Il was particularly fond of four of his female singers who were introduced as "lady conductors," Pulikovsky wrote.

And while the train has long been a source of intrigue, unlike some other aspects of the North Korean regime, it isn't entirely closed off from the public.

At Kim Jong Il's mausoleum in the North Korean capital of Pyongyang, the office car that he once used is preserved at a museum exhibit, complete with a desk and computer — proof, guides at the mausoleum say, according to the Associated Press, that the leaders never stopped working for their people.

Kim Jong Il also allowed cameras onto the train multiple times during his reign, which revealed flatscreen TVs mounted on the walls, laptop computers, conference rooms and satellite phones, which allowed him to be briefed on security issues.

It's not unusual for North Korea to send a leader overseas without any advance notice. In 2003, for example, Kim Jong Il made a trip to China, but it wasn't announced until days later, the AP said.

For Kim Jong Un, the surprise visit was his first known trip outside North Korea since he took power in 2011 — and a sign that he's making overtures to hold diplomatic talks ahead of a planned meeting between himself and President Donald Trump by May.

"This would be yet another sign that Kim is trying to push forward a diplomatic track that we have not seen from North Korea ever before," Victor Cha, senior adviser and Korea chair for the Center for Strategic and International Studies and an NBC News contributor, told "Today."

As for why he might choose to travel by train like his father — and grandfather — used to do, the experts don't know. Melvin, the Korea researcher, said Kim Jong Un has been building runways throughout the nation.

But Melvin speculated it might have been over security concerns that something could happen if Kim Jong Un were to fly.

"There is something to be said about security in North Korea right now. If Kim Jong Un were to become incapacitated or die in an accident, this would cause a tremendous amount of confusion and instability in North Korea," he said. "It's a unipolar political system, and a lot of the people in power are basically in power at the behest of Kim Jong Un. There's no mitigating institutions that power has been decentralized to maintain stability in that event."