According to a Korean Railfan,
Gyeongchun Line, which was incorporated in Seoul Metropolitan Subway last year, is planned to be extended to Yangyang in the eastern coast of Korean peninsula. But, Extended line between Chuncheon and Yangyang won't be incorporated in Seoul Metropolitan Subway.
Since Yangyang is near the border with North Korea, Gyeongchun Line will take an important role as a part of shortest rail link between Seoul and Kumgang-san swimming beach in North Korea, if trains on Donghae Bukbu Line were to run into North Korea.
But, the possibility is extremely little.
