Can confirm the Seoul Station City Air Terminal service only works for KE/OZ/Jeju, which is a bit of a bummer. The setup is pretty much identical (minus the faffing around with tickets, when you buy the ticket for the train you pick your time and seat), and gets you to ICN in ~45 mins direct (or 60-ish mins for the slower service), meaning you can get through security extremely quickly.

Very handy if you work in downtown. Check your bags in, buy your ticket and sort immigration out in the morning on your way to work, then whenever your flight is, take the train (already booked the ticket) and go straight through security at ICN. Couldn't be easier!