65 years of armistice has meant a demilitarized zone separating the nations, and heavy military presences on either side. As diplomatic progress continues, the re-connection of transportation underscores how quickly ties could be normalized.
This is good news for everybody, except apparently the Trump Administration, which is expressing "concern" about the rate of progress being made in Korean diplomacy, interfering with US plans to keep sanctions against North Korea tightly in place.
South Korea's President Moon has been increasingly open in his disagreement with the US on the matter, saying North Korea deserves some rewards for its progress made toward denuclearization.
Comment: The agreement signed between Kim and Trump specified the order in which events are to take place. The U.S. is already pretending this agreement doesn't exist. Peace treaty first, denuclearization after that. Luckily Moon and Kim are proceeding apace, despite American belligerence.