For south koreans, it's all about "can I go to gangnam within 1 hour?"

Most jobs, private tutors for kids, etc - are near/inside gangnam.

Any improvements for this metric (eg. a high-speed train) will fix the issue in no time

See "Gwanggyo new town" for example. It was a ghost-town until "new-bundang-line" (a high-speed train disguised as subway) made the trip to gangnam less than 40 min.