No surprise, then, that the appearance in Beijing on Monday of a deep green train with yellow piping that looked a lot like the one used exclusively by the ruling Kim family has generated so much speculation of a surprise visit by leader Kim Jong Un.
USA news agency Bloomberg has reported that the official believed to be visiting China is Kim Jong Un, chairman of the Workers' Party of Korea, citing information from three people with knowledge of the visit.
If the media reports turn out to be true, a visit by the North Korean leader - his first outside the reclusive country since coming to power in 2011 - or his sister, who has played an increasingly crucial role as a key diplomatic envoy from the North, would have broad diplomatic significance.
Chinese Foreign Ministry spokeswoman Hua Chunying, asked whether Kim or another North Korean official was visiting Beijing, said, "I know you're all very curious but I have no information on that".
Reports of heavy police reinforcements on the China-North Korea border and at a Beijing hotel popular with visiting dignitaries also fed the speculation.
Guides tell visitors that the carriage was used as a mobile office - proof, they insist, that the leaders toiled tirelessly for the people. Other media speculated it might have been Kim's sister Kim Yo Jong or the country's ceremonial head of state Kim Yong Nam.
According to South Korea's Chosun Ilbo newspaper, Mr Kim has not been seen in public since he met a South Korean delegation on March 5th amid reports he is gearing up for scheduled summits with South Korea and the US.
China-based North Korea-watchers scoured for clues to pin down the identity of the visitor.
North Korean news media has a history of publishing its former leader's China visits but only after he almost finished his trips, probably because of security reasons.
Mending his relationship with China would be huge for Kim ahead of the summits with South Korea and the U.S. It would make North Korea less isolated, and China could use its influence to help Kim get what he wants out of the upcoming diplomacy.
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Ties have been strained between Pyongyang and Beijing in recent times after the secretive state conducted a number of missile tests.
The younger Kim has not undertaken any official trip overseas since taking power following his father's death in December 2011.
Whoever it is, the move could also be part of a North Korean effort to gain leverage ahead of the planned talks with South Korean President Moon Jae-in and later U.S. President Donald Trump by showing that it has other options.
Un's train had also slowed train traffic in Northeast China and vehicles were also blocked just south of Tiananmen Square with police ordering newsmen to leave the area. Nippon TV showed footage of the train arriving in Beijing on Monday.
A special train believed to be carrying a North Korean VIP has left Beijing.
On the contrary, the official said, a summit with Xi would underscore Kim's standing as a world leader.
The Chinese capital is gripped by mystery that includes an armored train, amateur detectives and flurry of Internet searches for "fatty the third".
The North Korean leader is due to hold separate summits with South Korea in late April and the United States in May.
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