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.
Media reports about Kim's visit was in circulation following heavy police presence at the China-North Korea border and later at a Beijing hotel popular with foreign dignitaries, Hong Kong-based "South China Morning Post" reported.
When the late North Korean leader, Kim Jong-il, paid an informal visit to China in 2011, his travel was published by North Korean news outlets while he was on his way back to North Korea after finishing the seven-day trip. China's foreign ministry denied knowledge of any meeting with North Korea; in Seoul and Washington, officials said they had no conclusive information.
North Korea's relations with China, a traditional ally, have been strained since Kim purged several top-ranking officials close to Beijing, including his uncle, Jang Song Thaek.
North Korea, too, has cause to seek China's ear. "We don't know if they're necessarily true", he said.
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.
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.
Kim Jong Un is reportedly in China.
South Korean analysts were doubtful the visitor is Kim. He said that improving ties between North Korea and China would be a positive sign before the planned summits with US and South Korea.
Beijing is the main ally of secretive and isolated North Korea, as well as its biggest trading partner.
Chung Seong-chang, a senior fellow at the Sejong Institute specializing in North Korea affairs, agreed that it was Kim Jong-un who made the trip, not his younger sister.
CNN reported that the train vehicle had not left the secretive state since Kim became leader. Past glimpses of it, however, 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.
"Numerous other senior officials could and would have taken a private plane, possibly departing without any fanfare in Pyongyang and arranging to establish an unobtrusive arrival with the Chinese authorities", Madden said.
It follows Japanese media reports that a high-ranking Pyongyang official had apparently arrived in Beijing on Monday ahead of the summit with South Korea later in April.
Observers say that Kim's visit here was due as it is diplomatically inconceivable for him to meet Trump without ever meeting Xi, who is now set to become leader for life after the recent constitutional amendments removing the two term limits for the Chinese president.
"Never before have North Korean officials used that special train [seen in Beijing] except for Kim Jong-il", said Chung. South Korea's presidential office said it could not verify the reports either.
Neither North Korea nor China has yet to confirm any details about the reported visit.