RR21
I'll tell you how I did mine, of course, there are many other ways to accomplish the same outcome.
In my case I needed to put the waterfall directly above the track in the middle of a 90-degree curve. Not recommended, I did not want to do it that way, but real estate constraints dictated doing it this way.
Start with the tunnel. I built the tunnel with cinderblock. I "played" with several combinations and spacing to ensure that there was more than sufficient room inside the tunnel to allow the longest cars to pass through without hitting anything. After achieving the spacing I desired I dry stacked the cinder blocks, two blocks tall. I cut slots in the top block every six inches so that I could form a re-bar "roof support" system. (Think of a "giant staple" formed by the re-bar connecting the two walls.) On top of this I used cinderblock top cap bricks spanning the gap. I then put two layers of roofing felt. Since the water out fall box was to go above this I also put a layer of pond liner as I did not want any leaks getting to the rails. Atop this structure I sat the water box and started cementing stones to form the mountain. I included clay flowerpots in a few places to allow for plantings at a later date. I wanted (read Wife wanted) a rough rock mountain. So, cement and stone mainly form my mountain.
Others here use large diameter pipe, and dirt. They will have to speak of their techniques.
Things I wish I had known about BEFORE I built the tunnel.
1. Proper internal drainage: I did not allow for drainage of rainwater that would accumulate inside the tunnel. I had to open things up a bit to install some drain piping to remove the standing water.
2. Texturing the inside of the tunnel: After watching some of Ray Dunakin's videos (he has some fantastic mountains and tunnels) and looking at my own videos with the trains running through the mountains, bare cinderblock walls inside the tunnel detract form the overall effect. I'm planning on reaching inside the tunnel to spread some quickset mortar to put some sort of texture inside. LESSON LEARNED: texture the inside walls before putting the roof on your tunnel.
Here is a glimpse of my mountain.
