Thank you for the nice report.
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I was in the area (Gyeongju) for a couple of days during Chuseok to meet a Korean friend. Went by express bus though as I was coming from the South West.

Asiana don't do business on domestic routes, I believe, but KAL still has business seats for a very modest premium(?). Pohang being a pure business destination (Posco etc. ) I would expect a big demand for business seats on the Gimpo-bound routes. Can you get direct flights to Jeju Island from Pohang?

Btw, how are the coach class seats on the KTX? I have wanted to take the train several times in Korea but for various reasons have resorted to flying or driving in the end. If the leg space is as limited as the french TGV ( isn't the KTX a TGV-based train? ) I might skip it all together or try first class. What kind of food & drinks do they serve during the trip?