I'm surprised there has been little chatter on the net about this accident (none in the rail-oriented forums I frequent). Perhaps the ferry disaster is overshadowing everything. Anyway, this article has the most info about the possible cause, though it is not 100% accurate, either because of translation issues, or reporter ignorance of railway technology and operation:
*make that five seconds
*lol
http://koreajoongangdaily.joins.com/news/article/Article.aspx?aid=2988786
A better written article than the one above:
http://koreajoongangdaily.joins.com/news/article/Article.aspx?aid=2988852
Hmm, maybe Seoul Metro officials would benefit with a visit to Tokyo Metro's Fukutoshin Line, where rolling stock is fitted with five different signaling systems: CS-ATC, ATO, ATC-P, Tobu ATS, and Seibu ATS- that work smoothly and safely.