Typically, when you build a mountian with a tunnel and want to line the tunnel with rock work you do it before the mountain is built. Well when I started building scenery some years ago, that wasn't something I considered. Today, I went ahead and got started with the tunnel liner for the tunnel passing through South Mountain using polyurethane foam and fiberglass screen. The first two photos picture the tunnel as it currently exists. Just a black hole. I want to install a tunnel liner that will give the appearance that my trains are actually passing through a tunnel blasted through rock. The tape measure was being used to determine how long the liner should be inside the mountain. I decided 14". While I didn't install a tunnel liner prior to building the mountain, at least I put in an access hole underneath the mountain that will allow me to standup inside it. That will allow me to work at both ends of the tunnel. It's still going to be a challenge!
This third photo shows partial assembly of the liner pieces made with the polyurethane foam and fiber glass window screen material (screen foam).
The final photo shows the basic liner that was completed today. Along with some of the materials used. Off to the left is the template made out of scap 2'X4' material. It was used to form the "screen foam" strips into the proper contour for the tunnel liner.
The liner at this stage is amazingly lightweight. The next steps will be to cast the rocks for use in lining the interior of the tunnel. This project is based on information garnered from Eric Siegel's website Eric's Trains and with materials and instructions from Bragdon Enterprises. I'll continue to post the progress and hope to have more on my website later this week.
You can visit my website Bo's Trains at http://www.bostrains.com
Bo 
