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I can't remember but I think you go to a nearby village and (inevitably) do some side quests in exchange for the route to the ice temple.

To elaborate on what's stated above, there's this train enthusiast named Ferrus who lives in the eastern half of the Snow Realm. He lives in this place called Wellspring Station. First, you go there, and there will be a map with three dots. Ferrus is always at one of those three dots taking pictures, though which one he's at is completely random. Just pay a visit to each spot and stop your train next to him when you find him. Just tell him you're trying to get to the temple and he'll give you a map w/ the right path to it. Blow into the microphone to get the dust of the map and copy the route onto your Rail Map.