I’ll go on the subway.
Tokyo's subway is extremely complex.
What a relief! I thought I missed the subway train.
The ground below iron is iron under the ground. It isn't a deposit of iron, though. It's man-made. And what's made of iron? Trains. In this case, because it's underground, it's a subway. Depending on where you live, you might call it the Underground, the Tube, or the Metro
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.