I've stared at these subway signboards many times over the past few years, and only yesterday did I suddenly realize how it works. I guess I'm slow. Along the bottom row of the sign, usually it will be blank or have text belonging to the message. But, when the train gets close, suddenly an image of a train will appear. The train slowly crawls along the bottom of the sign. What I realized was that the sign frame is labeled with the names of the two previous stations. In this case, I am at the Omokgyo Station, and the train is about to arrive. The two previous stations in this case are the Mokdong and Shinjeong stations. So you can tell clearly how far away the train is as it approaches your station! Pretty cool. Sorry for the poor quality, it was taken with my cell phone camera.