Just over a year ago I installed an HO scale train around a ledge in the great room of my house. This year, I thought it needed a mountain to go with it. In this Instructable, I'll explain how I made the mountain so you can make your own.

What you'll need:

3/4" plywood
1/2" plywood
chicken wire
Several cans of "Great Stuff" foam insulation
Latex paint
Pull saw
Newspaper
Nail / staple gun
Model railroad trees
Fake snow

Step 1: Build the Frame

I decided to place my mountain above the fireplace in our great room, but didn't want to block the windows. It's about 65" wide, 13" tall, and 12" deep.

I designed the mountain to fit over the existing trackbed and provide a tunnel for the train to go through. For the base, I simply cut a few pieces of 1/2" plywood. To help support t he chicken wire and to add to the height of the mountain, I cut ribs out of 3/4" plywood and screwed them to the base.

Most important was testing the train and track to ensure it would fit through the tunnel without scraping the side.

Step 2: Wrap With Chicken Wire, Stuff With Newspaper

Unfortunately, I didn't take a decent picture of how to attach the chicken wire. I laid the chicken wire over the ribbed frame and then stapled it down with brad nails. It was easy to form the wire into the shape that I wanted, providing plenty of "rock-like" faces. If you decide to do a tunnel, be sure to completely cover the entrance; you can cut it open later. Use wire cutters to trim off the excess chicken wire.

Once the shape of the mountain is complete, stuff the form with newspaper. This will give the spray foam something to stick to. Use as much newspaper as you can and don't worry if you cover the space for the tunnel -- you will be removing the newspaper once the foam dries.

Step 3: Spray Foam

Important: Apply the foam in a well ventilated area!

I ended up using "Great Stuff" spray foam intended to fill small (1-2") gaps. I tried the foam for larger gaps, but it didn't seem to make much of a difference. In fact, I thought it was harder to work with. Each can cost about $2.50 and for the size of this mountain, I went through 7 cans total.

Hold the can close to the wire frame and begin applying it in rows. The foam will expand slightly after application so you don't have to pile it up. You will need more cans of spray foam then you originally estimated. It looks like it goes a long way, but unfortunately does not. Keep in mind that once you open the can it will need to be used completely.

The outside of the foam will harden in a couple of hours and become completely solid in about 24 hours. Don't rush it --- you don't want to cut into the gooey center.

Step 4: Sculpting Your Masterpiece

Once the spray foam has completely hardened, you can begin carving. As I mentioned in the previous step, be patient. The foam will expand a lot as it dries so you'll have plenty of area to carve on.

I used a fine-toothed pull saw to carve the faces of the rock. It worked well because it was long enough to cut wide faces. Work your way across the entire mountain cutting off the rounded "lava-like" sections of spray foam. Be careful not to cut too deep -- you don't want the chicken wire to show through.

I cut horizontal and vertical ridges into the rock face to simulate a real mountain.

Step 5: Paint

After I was satisfied with the way the rock faces look, I applied layer of light grey latex paint. The foam soaked up the paint so it took several coats before it looked right. After the first coat was dry, I installed the mountain over the fireplace to ensure the train would make it through the tunnel.

The first coat of paint helped to highlight areas that needed work. I used an additional can of spray foam to fill in holes and remake areas that didn't look quite right. After the foam dried, I used the pull saw to trim off the rounded areas.

Using standard acrylic paints (from Hobby Lobby), I mixed up some dark grey and brown colors to feed through my airbrush. The first couple of passes didn't look too good -- the mountain looked as if it had a bad camouflage pattern. It took several coats before it started to look like actual rock.

One technique that I found useful was to "force" light and shadow. That is, I applied a lighter color to the rock faces that were more horizontal and a darker color in the crevices and vertical slopes.

Many model railroad websites recommended watering down the acrylic paint and then applying with a spray bottle. Unfortunately, that technique didn't work well with this type of foam as it was not as dense as hobby foam.

Step 6: Finishing Touches

I decided to add fake snow rather than painting it directly onto the rock. The nice thing about the fake snow is that it acts like actual snow and gathers on the rock faces and in crevices like natural snow. Plus, since I didn't paint the snow on, I can now change the mountain with the seasons.

I found some snow covered trees that my wife had purchased years ago for a Christmas village, removed the bases, and stuck them into the foam. I also placed trees along side the mountain to round out the effect.

I hope this Instructable has inspired you to run off and move your own mountains. If you have any questions, post them in the comments and I will happily answer them.

Thanks!