Learn French: Le métro parisien est rapide. - The Parisian subway is fast.

Because le métro is a masculine noun! The spelling parisien is the base masculine form. If you were describing a feminine noun, like la gare (the train station), you would apply the rule from this lesson: double the final consonant and add an -e to make it la gare parisienne.

Why isn't parisien capitalized like "Parisian" is in English?

In French, adjectives of nationality, origin, or regional identity are never capitalized. You only capitalize these words when they are used as nouns to refer to the people themselves, such as un Parisien (a Parisian man).

Why does parisien come after le métro instead of before it?

Most French adjectives are placed after the noun they describe. This is a strict rule for adjectives describing origin, geography, or nationality, which is why parisien comes directly after le métro.

Does the adjective rapide need to change to match le métro?

It already matches! Le métro is masculine, and the masculine form of "fast" is rapide. Because it naturally ends in an -e, the feminine form is also exactly the same. You only need to change it by adding an -s if the noun is plural.