Answer :

passing is the present participle

Identify the present participle in the sentence.

The cars passing by are filled with commuters.

The present participle form is formed by the  verb + ing, and its use depends on its placement.  

In the case of the provided sentence, the verb would be "pass"+ing = "passing". Furthermore, the present participle in the sentence has the function of a verbal noun: "the cars passing by".