metro **
tube , ***
underground
subway system
subway *

It is the 15th increase in the 100-year history of the subway system.

And there's a lot that can be done with the subway system.

He was the first person to be killed on the subway system this year.

I couldn't figure out the subway system on how to get there.

Chicago had a subway system, and I could get around.

The subway system has hit a century of home runs.

"No one knows this subway system better than we do."

And the story of the new subway system is wonderful.

Much of the focus will be on the subway system.

About seven million people a day ride the subway system.