I think definiteness doesn't need to be a shared context. "I went to the the train station to pick up a map on my way here". "Great, thanks!" It was a specifi

I think definiteness doesn't need to be a shared context. "I went to the the train station to pick up a map on my way here". "Great, thanks!" It was a specific train station that I chose intentionally or visit regularly, but it's not relevant to the conversation which one. If I were to use "a train station" then it implies I didn't care which train station it was.