North Korean leader Kim Jong-un’s special train arrived in the central Chinese town of Changsha on Monday on its way to Vietnam for a summit with U.S. President Donald Trump later this week, a source said. The train arrived in Changsha of Hunan Province around 1:10 p.m. after passing through the city of Wuhan earlier in the morning and heading south, according to the source.

Security was tightened with officials blocking roads around the Changsha train station before the arrival. It is not clear whether Kim’s train already passed the station or stopped for a while, with some speculating that it might have stayed there for refueling. Though it is still unclear which route Kim’s special train will take from there, it is expected to continue to run south to Nanning and Pingxiang, the Chinese town near the border with northern Vietnam. Kim’s train is expected to cross the border into the northern Vietnamese border town of Dong Dang. From there, the North Korean leader will likely travel by car to Hanoi. The route is still regarded as the shortest way of moving from Pyongyang to Vietnam. Experts say that there is a possibility that the train could pass the southern Chinese city of Guangzhou.

Kim’s train departed Pyongyang around 5 p.m. on Saturday and reportedly reached the northeastern Chinese port city of Tianjin on Sunday afternoon. Contrary to wide expectations, the train did not stop in Beijing, though there is a possibility that it could stop there on its way back. The train is expected to arrive in Dong Dang on Tuesday. Vietnamese media outlets reported that all traffic will be banned on a section of highway linking the city to Hanoi between 6 a.m. and 2 p.m. on Tuesday, and that the Vietnamese military carried out mine detection on the road. Train services arriving at the Dong Dang station were halted on Sunday and will remain suspended until Saturday, according to local news reports. That raises the possibility of Kim using the train again on his way back to Pyongyang.

Security has been tightened with more patrolling around the Pingxiang station, the last Chinese town Kim’s train is expected to pass before crossing into Vietnamese territory. As the train will likely pass the station early Tuesday, it is possible that the once-a-day service linking Pingxiang and Hanoi could be suspended. Kim is scheduled to hold his second summit with Trump on Wednesday and Thursday. He is expected to arrive in Hanoi on Tuesday, and Trump is to leave for the Hanoi summit on Monday (local time). Kim used a rented Chinese plane to travel to Singapore for his first summit with Trump last June. At that summit, Kim and Trump agreed to the complete denuclearization of the Korean Peninsula in exchange for security guarantees from the U.S. The two are expected to focus this week on concrete denuclearization steps by the North and corresponding measures by Washington, possibly including easing sanctions and improving bilateral relations.