地下鉄

In a sentence

• 私は地下鉄で学校へ行きます。

  I go to school by subway.

  Here, '地下鉄' is used to refer to the mode of transport, the subway, emphasizing the means of traveling to school.

• 東京の地下鉄はとても便利です。

  The subway system in Tokyo is very convenient.

  In this sentence, '地下鉄' refers to the entirety of the subway network, focusing on its attributes, like convenience.

• 地下鉄の駅で友達に会いました。

  I met my friend at the subway station.

  Here, '地下鉄' is used as a descriptor, modifying '駅' (station) to specify the location as being a subway station.