Fog only in specific areas.

I’m making a subway map, and two stations are in a straight line from one another. I can see the light from the other station in the tunnel, even at the start. But, i set up a fog controller, and it also does it in th station, which i don’t want. Is there a way to have the fog begin at the entrance to the tunnel, and end at the other end, and vice versa?

Func_smokevolume

Thanks Firegod, that got just the effect i was looking for after a little tweaking.