This is an example of why the South Koreans need the US to agree to drop sanctions before they can begin any projects with North Korea:
After the two Koreas agreed on Tuesday to connect their railroads and work together on modernizing the North’s infrastructure, one question looms: Is the project even possible?
If South Korea were to provide North Korea with vehicles, machinery and other equipment for track construction, it would violate United Nations Security Council Resolution 2397, which prohibits the export of industrial equipment to the North. The sanctions, passed unanimously on Dec. 22, 2017, came after the North tested an intercontinental ballistic missile on Nov. 29 of last year.
During the meeting between officials from the two Koreas on Tuesday, both sides agreed to boost cooperation in modernizing North Korea’s rails, stating the end goal was “balanced development of the national economy and co-prosperity.”
South Korean President Moon Jae-in has espoused a grand vision of creating a single market with the two Koreas to lay the foundation for unification, job creation and economic growth in both countries. [Joong Ang Ilbo]
You can read much more at the link, but I expect that the Kim regime will continue to play nice in order to convince President Trump to drop sanctions for little to nothing in return. I would not be surprised if the Kim regime decides to continue to play nice until after the mid-term elections in the US. This will give them an assessment of the landscape in the US moving forward in 2019.