Pictures of the Year: A Picture and its Story
The monitoring project, 38 North, said in its report on Saturday that the train was parked at the "leadership station" in Wonsan on April 21 and April 23. The station is reserved for the use of the Kim family, it said.
Though the group said it was probably Kim Jong Un's train, Reuters has not been able to confirm that independently, or whether he was in Wonsan.
"The train's presence does not prove the whereabouts of the North Korean leader or indicate anything about his health but it does lend weight to reports that Kim is staying at an elite area on the country's eastern coast," the report said.
Speculation about Kim's health first arose due to his absence from the anniversary of the birthday of North Korea's founding father and Kim's grandfather, Kim Il Sung, on April 15.
China has dispatched a team to North Korea including medical experts to advise on Kim Jong Un, according to three people familiar with the situation.
Reporting from inside North Korea is notoriously difficult because of tight controls on information.
A third-generation hereditary leader who came to power after his father's death in 2011, Kim Jong Un has no clear successor in a nuclear-armed country, which could present major international risk.
Daily NK, a Seoul-based website that reports on North Korea, cited one unnamed source in North Korea on Monday as saying that Kim had undergone medical treatment in the resort county of Hyangsan north of the capital Pyongyang.
It said that Kim was recovering after undergoing a cardiovascular procedure on April 12.
Since then, multiple South Korean media reports have cited unnamed sources this week saying that Kim might be staying in the Wonsan area.
On Friday, local news agency Newsis cited South Korean intelligence sources as reporting that a special train for Kim's use had been seen in Wonsan, while Kim's private plane remained in Pyongyang.
Newsis reported Kim may be sheltering from COVID-19, the respiratory disease caused by the novel coronavirus.
North Korea's state media last reported on Kim's whereabouts when he presided over a meeting on April 11.
Kim, believed to be 36, has disappeared from coverage in North Korean state media before. In 2014, he vanished for more than a month and North Korean state TV later showed him walking with a limp. Speculation about his health has been fanned by his heavy smoking, apparent weight gain since taking power and family history of cardiovascular problems.
