In the early days of model railroading, most mountains and tunnels were made with mounds of crumpled newspaper and plaster. It was messy, inefficient, heavy and cracked easily. Today many modelers prefer working with foam sheets and light plastercloths like those used to set broken limbs. You can use extruded foam insulation used in home construction or sheets made especially for model railroading to create large mountain ranges and smooth, even tunnels in a fraction of the time of the old methods.
Building the mountain
Measure the area you have to work with and determine a manageable length, width and height of the mountain tunnel you wish to model.
Mark the edges of the bottom layer with a marker and cut out the shape with a hobby knife.
Create the next layer of the mountain side with another piece of foam, marking and cutting in the same manner, continuing the process until you have enough layers to create the height you want. Generally speaking, each progressive layer will be a little smaller than the last, leading toward a ridge or peak.
Stack the layers one by one, temporarily securing it with finishing nails.
Determine the entrance and exit points for your tunnel and mark the edges of the openings on each side, leaving at least 1/2 inch clearance on each side and the top to make sure the train can move through the tunnel.
Remove the top part of the mountain which will form the roof of the tunnel and set aside.
Mark lines from the entrance point to the exit point on each remaining layer and cut along those lines with a hobby knife in order to create the tunnel. You will end up with three pieces, discarding the center piece where the train will eventually run.
Glue the bottom layer of the mountain to the layout board with white glue. A few finishing nails can be useful here too.
Lay in the cork roadbed and the flexible track with rail nails and test the line with an engine before gluing in place.
Add successive layers of the mountain, securing with a few thin lines of glue and finishing nails in between each stage.
Dip a plastercloth into a pan of water and lay it across the foam vertically.
Coat the mountain with plastercloths, smoothing out the texture with your fingers as you go, and allow a drying time of 24 hours before proceeding.
Painting the mountain
Select a light earth tone acrylic paint, like light gray or tan, and cover the entire mountain as a base coat.
Mix solutions of 1 part paint to 16 parts water in several shades of earth tones. This is called "a wash."
Apply the washes randomly in horizontal strokes, allowing some areas to blend to create striations and color variations.
Mix a few drops of India ink into a cup of distilled water to create a shadow wash and brush liberally over the surface.
Allow a drying time of at least 24 hours.
You may want to add commercially produced tunnel entrances when the mountain is complete. They will make the layout look better and ensure proper clearance for your trains.
If you want more texture for the mountain, tape down a little crumpled paper before applying the plaster wraps.
You may want include a re-railer track inside the tunnel to avoid derailments and leave an open area in the back for access just in case.
A piece of paper bent into a curve and inserted into the tunnel at the entrance and exit can help create the illusion of a shaped passage.
Do not use a hot foam cutter with standard foam insulation. This releases highly toxic gases. Hobby stores often have special foam that can be used with foam cutters, however.
Do not bother with compressed bead foam sheets. They will not cut well and the loose beads will create a mess and stick to everything.