Tunnel Flush

http://www.railpictures.net/viewphoto.php?id=70563

What is a Tunnel Flush? Surfed for the answer and did not find it.

In short, those diesels you have a picture of smoke up the tunnel. Tunnel flush is the ventilation system that flushes out the smoke. There are different rules for different tunnels.

Thank You !

If you have a copy of Kalmbach's 'Trains On Location-Stevens Pass' video there is a segment on tunnel flushing. A railroad electrician explains how they clear out the Cascade Tunnel. It's pretty impressive when you see the size and force of the cloud that comes out of the tunnel when they open the portal door.