I am replacing the one track tunnel under my waterfall(has a leak-so have to tear it out for repairs anyway). I am putting in a two track tunnel to cure a bottleneck and thus create two separate loops. Sides are made of pressure treated 2x12's ripped down to 10 1/2" by 4'. My wife used to have a sign business and we had some scraps of double layer aluminum sign board with plastic honeycomb in between for the top and bottom. So more weather proof material. The portals were made with plastic brick molding pieces I had left over. They were ripped down to the sizes I wanted to use. Sort of made this up as I went along, combining some ideas I saw of different portals on the internet. The inside of the tunnel has some thin strips to give an idea of the beams inside. I kept them thin so I can get large locos through it.