The last defection involving North Koreans occurred in May when two civilians aboard a small boat defected to the South across the Yellow Sea.
The six-carriage train is transporting 28 South Koreans including railway engineers and other personnel, and carrying 55 tonnes of fuel and an electricity generator.
News on the summit timeline comes despite the cancellation of recently planned talks between U.S. Secretary of State Mike Pompeo and DPRK counterpart Kim Yong Chol, as well as the retirement of CIA Korea Mission Center head Andrew Kim.
There is a passenger coach, a sleeping coach, an office coach and a wagon loaded with water for showers and laundry.
At Panmun Station across the border, a North Korean engine took over to take them further north.
They will travel some 2,600 kilometres (1,600 miles) on railway tracks together, the transport ministry said. "We will maintain close consultation with related nations so that the project to connect the South and North's railways could proceed with worldwide support". -North Korea nuclear diplomacy.
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"We're not having missiles launched, there haven't been any nuclear tests", Pompeo noted, citing a change in behavior from North Korea, which conducted a series of missile tests in 2017 that alarmed its closest neighbors - and had the USA concerned that Pyongyang was developing the capability to strike any site on the continental U.S.
According to related reports, Seoul and Pyongyang had agreed on Kim making the trip in mid-December, but the North requested the trip to be postponed due to security concerns.
The two Koreas have made significant progress in improving their relationship over the past year.
Moon said a second summit between Kim and Trump will prove to be the "most critical moment" for North Korea's denuclearisation. From Dec. 8 to 17, the Koreas will inspect an 800-kilometer (497 mile) railway section along the country's eastern coast, stretching from an area near the scenic Diamond Mountain to a riverside station near the North's border with Russian Federation.
But differences have emerged between Washington and Seoul on how to proceed with Mr. Kim, as the dovish Mr. Moon has long favoured engagement with the North.