The North Korean train that arrived in Beijing this week looked remarkably similar to one used by the Norths late leader Kim Jong Il, its olive-green exterior and yellow stripe prompting intense speculation his son and successor Kim Jong Un was on board.
According to the official North Korean account, he was on a train for a "field guidance" visit in 2011 when he died of a heart attack.
The Kims reportedly have several almost identical special trains, made by a factory in Pyongyang.
They are typically composed of two engines and 17 to 21 cars, and travel at no more than 60 kilometres (38 miles) per hour.
Protected by armour plating, they are equipped with satellite phones and television screens, and include conference rooms, bedrooms and reception facilities, the Chosun daily said.
They reportedly carry armoured vehicles and small helicopters for emergencies.
For security reasons, three identical trains were deployed for every overseas trip by Kim Jong Il and Kim Il Sung, the current leader's grandfather, news reports in Seoul said.
The train seen in Beijing this week looked particularly similar to one used during Kim Jong Il's 2010 trip to the Chinese capital, according to the Joongang daily.
Japan's Kyodo news agency reported the train's departure from a Beijing station Tuesday, with the identity of its passengers still unconfirmed.
The carriages used by both late leaders are now on display at the Kumsusan Memorial Palace in Pyongyang, where their bodies lie in state -- and a Macintosh computer stands on Kim Jong Il's desk.