First, its worth pointing out exactly where this tool resides. In the middle left hand menu, under "misc" and called "Tunnel Occlusion Volume"
OK, the first thing we need to do for the purposes of this tutorial is create a basic tunnel (Fig 1) and with no modifications added, Fig 2 shows the view from inside the cab of a Class 47 which doesn't even have lights on and is at the mid point of a 800m tunnel. I don't think many could argue that it is far too bright in there.
Although this will be entirely user specific, I have found that placing two of these in the same place gives the best effect. This is easily done by simply copy/pasting your already sized tool before placing inside, and then placing both in the same location.
This final shot shows the finished result once two tools have been placed. Again, taken from the Class 47 cab with no lights on, the shot is taken from exactly the same location as before. As you can see, much more darker and much more realistic.