How about Asahiyama Zoo Train （旭山動物園号） from Sapporo （札幌） to Asahikawa （旭川）?
Isn't it fun to use such a cute train with your friends or family members?
Personally I also recommend Lupin the Third Wrapping Train （ルパン三世ラッピングトレイン）↓
(See the details→★)
This special train runs in the Hanasaki Line （花咲線） located in the eastern part of Hokkaido.
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