地铁 is called differently in different countries.
E.g.
In England, people usually call it tube or underground.
In US, it is usually called subway.
In France and Canada, they use the word, metro.
So it depends where you are living at the moment, but most of the people in China will use the word, subway, because we are using US English across our country.
If subways are already existed in the cities around that city, you should say
Your city is planning to construct a junction for subway.
or
Subway is planning to extend its service to your city.
If not, you should say
Your city is planning to construct subways.
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