After stepping off the train in a black Mao-style suit, Kim shook hands with officials and slowly walked out of the station, beaming a smile and waving to school kids. Kim soon headed to the capital city of Hanoi in a black limousine. The 170-kilometer road trip is expected to take a few hours.

Kim is scheduled to hold a summit with Trump on Wednesday and Thursday.

Trump is flying from Washington DC and is due to land at Noi Bai International Airport in the Vietnamese capital Tuesday night.

Trump expressed optimism about his upcoming bargaining with Kim.

"Heading over to Vietnam for my meeting with Kim Jong Un. Looking forward to a very productive Summit!" he tweeted aboard Air Force One.

Before departing for Hanoi, he also said, "We want denuclearization, and I think he'll have a country that will set a lot of records for speed in terms of an economy."

The two leaders are scheduled to begin a two-day summit with a one-on-one meeting and a social dinner, to be joined by their top aides, in Hanoi Wednesday night. Secretary of State Mike Pompeo is already in Hanoi.

The initial meeting is expected to set the tone for this week's event, a watershed in the "top-down" diplomatic process.

Kim and Trump will have follow-up talks the next day in a bid to flesh out the details of their Singapore accord eight months earlier, in which they agreed to work together for "complete denuclearization," transformation of Pyongyang-Washington relations and a peace regime.

There has been little substantive progress, however, in the process amid disputes over what denuclearization measures by the North and corresponding steps by the US should be taken and their sequencing.

The summit location remains unannounced amid speculation that the Sofitel Legend Metropole hotel right next to the government guesthouse, where North Korea's advance team is staying, has been chosen.

Kim is expected to stay at the Melia Hanoi hotel in the city center. The US had planned to set up a press room for White House correspondents there but decided to relocate it to another hotel.

One focus will be whether they can produce a joint declaration calling for concrete steps toward denuclearization by North Korea and corresponding US measures that satisfy Pyongyang.

The North's main Yongbyon nuclear complex is seen as a major card the North Korean leader could play in return for such incentives as the proclamation of a formal end to the 1950-53 Korean War and the continued suspension of major combined military exercises between the US and South Korea.

Washington's priority is to freeze the North's weapons of mass destruction program and delivery means, according to US officials. They also hope for verifiable dismantlement of the Yongbyon facilities and a clear denuclearization road map.

Posting liaison officers to each other's capitals is also being considered. As he did in Singapore, Trump, a former businessman, is likely to dangle the carrot of a bright future for the impoverished nation if it ceases its nuclear weapons and missile programs.

Kim is expected to call on the US to let its guard down and relax sanctions on Pyongyang.

The format in which they could announce their agreement, if they reach one, is also grabbing attention. Observers do not rule out the possibility of a joint press conference, a routine post-summit ceremony for Trump and other foreign leaders. (Yonhap)