1. There weren't any seats in the KTX.
2. There weren't any seats on the KTX.
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Do we have to use 'in' or 'on' in this sentence? [The KTX is a train which goes faster.]
In this case, you would use the preposition "on" in both sentences.
When referring to transportation, the preposition "on" is typically used to indicate that someone or something is physically on or inside a vehicle.
So, you would say "There weren't any seats on the KTX" to indicate that there were no available seats within the train itself.