Bloomberg news said the visitor was North Korea’s leader Kim Jong-un himself, citing three unnamed sources.
China is Pyongyang’s only major ally – but ties have been strained by tension over the North’s nuclear ambitions.
There has been no official comment on the reports, which were not covered by Chinese or North Korean state media. But if a visit is indeed under way, it would be seen as a significant diplomatic development.
“The government is closely communicating with relevant countries and monitoring the situation,” a South Korean government spokesman said on Monday.
Footage of the train from the Tokyo-based Nippon News Network showed a green carriage with yellow horizontal lines. The channel said it resembled one used by Mr Kim’s father and predecessor Kim Jong-il when he visited Beijing in 2011.
Kim Jong-il’s trips to China were only confirmed after his departure.
The manager of a shop outside Beijing railway station described seeing “unusual” scenes on Monday afternoon.
“There were a lot of police officers outside and along the road in front of the station. The station was blocked inside,” he told the AFP news agency.
Police also ushered tourists out of Beijing’s Tiananmen Square, according to Reuters, which usually signals a high-level meeting in the Great Hall of the People there.
A motorcade with a police escort was spotted driving away from the venue, the agency reports.
Earlier in March, North Korea’s foreign minister attended talks in Stockholm with Sweden’s Prime Minister Stefan Lofven, ahead of a possible, but as yet unarranged, meeting between US President Donald Trump and Kim Jong-un.