an electric railway operating below the surface of the ground (usually in a city); "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'" 地下を走る電車（通常都市で）。
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【上位】 line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight 乗客または貨物輸送をする電車の輸送のシステムの操作に関与する商業的な組織である路線。
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【上位】 a passageway through or under something, usually underground (especially one for trains or cars); "the tunnel reduced congestion at that intersection" 何かを通り抜けて、あるいはその下を通る道で通常は地下にあるもの（特に列車や車用）。
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