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Both Kim Jong Il and Kim Il Sung, the current leader's father and grandfather respectively, traveled by heavily armored train when they visited their allies in China and Russian Federation.

Former US Secretary of Defense William Cohen has said he was skeptical about a planned meeting between President Donald Trump and North Korean leader Kim Jong Un, according to a CNBC report on Monday.

It has sparked speculation a senior delegation - or even leader Kim Jong-un himself - might have been on board, particularly since the green-and-yellow-train appeared to resemble one used by Mr Kim's late father Kim Jong Il to travel to Beijing in 2011.

In Beijing, a vehicle with a diplomatic license plate of the North Korean Embassy was spotted Monday at the Great Hall of the People, a state building used for activities by the government and the ruling Communist Party of China. Having Kim choose the United States and Trump-which are openly hostile toward North Korea-over China and Xi Jinping for his first foreign foray would have been embarrassing. The train is thought to be the same one Kim took to Beijing in 2010. China is North Korea's treaty ally, and will likely try to inject its own interests into the negotiations, according to Sun.

"Starting at 10 p.m. Dandong Station was completely blocked, and two trains passed at an interval of about 20 to 40 minutes", Daily NK's source said.

Jacques deLisle, who teaches Chinese law and politics at the University of Pennsylvania Law School, added: "The Chinese want a sense of what Kim expects or is seeking in the meeting with the USA, if it ever occurs, as well as their attempt, of course, to influence it".

As North Korea's leader, Kim has overseen an extremely rough patch in relations with Beijing.

Police also removed all tourists from Tiananmen Square at the same time as the train's arrival. The train's appearance coincided with a notice announcing the delay of trains leaving Beijing for the northeast. Kim Jong Un has refused offers to visit Beijing in the past, but has recently changed his tone regarding diplomacy and face-to-face meetings.

However, he is not known to have travelled outside the country since assuming power in late 2011 after his father's death.

Other diplomatic sources said they were aware of the speculation that Kim was visiting but were not able to immediately confirm it.