This is the 5784th most frequent Russian word.
проходящего
"Passing" or "transient."
Here, 'проходящего' refers to 'passing by', describing the action of the person related to the verb 'проходить'.
Я видел проходящего мимо человека.
I saw a person passing by.
Here, 'проходящего' describes the route's characteristic of passing through, derived from 'проходить' meaning to pass.
Самолет направлен в город, маршрут проходящего через горы.
The plane is directed to the city, with a route passing through the mountains.
In this instance, 'проходящего' refers to the train that's currently passing by, used to describe its action derived from 'проходить'.