The Gold Shields are necessary to unlock some cities (e.g. Haikang). You can get them after completing a city with 5 badges and closing it. You can close it by traveling to another city and going to the list of stations to close the previous one.

 

Remember that once a city is completed and closed, you won't be able to come back to it. 

 

You can get more info about badges by tapping on the cities menu (bottom right orange button with a badge) and then on the "?" sign.