The Underground, The Metro, The Tube
Noun
ちかてつ
地下鉄に乗って行きます。
I’ll go on the subway.
東京の地下鉄はかなりふくざつです。
Tokyo's subway is extremely complex.
ああ、よかった！地下鉄に乗り遅れちゃうかと思ったよ。
What a relief! I thought I missed the subway train.
The iron below ground isn't a deposit of iron. It's man made. And what is made of iron? Trains and subways. In this case, because it's underground, it's definitely a subway, though.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.