When you transfer from/to AREX(Airport Express), don't be afraid to passing transfer gate. It doesn't charge you any additional fee. Remind that Hongik Univ. Station is always busy.
try the mushroomnoodle shop with the thick noodles ala ban mian. leave some soup for the rice at the end of the meal. down the street from sangdang building opposite peony cafe.
Exit 9 is the busiest exit I've ever seen. To avoid lining up in the mess. Walk straight thru the arcade/mini mall and up the stairs. Much fast way out of the metro.
If you are trying to get to Hongdae, you should exit through Gate #8! Everyone tries to enter and leave though Gate #9 and it’s a nightmare traffic jam there.
This may be the fastest and most reliable way to commute between Seoul station and Inchon airport~ 43 minutes and 13000 won! Cheaper train (slow train) takes 53 minutes but cost only about 4000 won!