Last Feb, I took ordinary train from Hongdae to Incheon airport, without several stops (less than 3), it only took us 50 mins. The cost is much lower than Arex train departing from Seoul station. The cost is less than 3900 won and child is half price (older than 6). I will definitely take this one again. So efficient and cheap. There's elevator from Hongdae metro walking to Hongdae train station.