Metro is a zone that holds multiple metro levels - all sectioned within colour-coded zones.

You can play any unlocked Metro level as long as you have high enough Combat Power (CP) and you have enough Metro Tickets.

On the top right corner of your screen you can see your CP and current amount of Metro Tickets

Each Metro level shows how much CP do you need to have, as well as the required amount of Metro Tickets to play it.

You can replay any Metro Level as many times you want.

All resources/breakable items will respawn every time you enter a level anew.

You can see how many resources of a specific type are in each level by tapping the Level Resources Info Button