I have made most of the scenery and rocks on my layout from standard "pink" foam insulation.  It can be carved into almost anything.  When painted it can look like stone wall, rock, brick even siding.  My technique is to carve and break the foam into rock like shapes.  Some, I"distress"  with a wire brush.  Be creative!  I look at real rocks and try to duplicate the randomness of nature.  The foam chunks are glued together with liquid nails.  Once it has set, I start painting.  I use acrylic craft paint right out of the jar.  I blend two or three colors to keep it natural.  Undersides and crevices get a little dark gray to enhance the depth.  After the paint is dry I mist everything with a "black water" spray. (water, 2-3 drops detergent and india ink)  This brings out the relief in the carving and scratches.  Last is some white dry-brushed on the tips to bring out the highlights.   Experiment.  If you do not like the result…..repaint and start over.

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These are the basic tools.  The knife must be kept sharp or the foam snags.

The trestle bases are carved from pieces of rough shaped foam

Painted base in place among fallen rocks

Eagle Creek Railroad  | Track Plan  & Construction | Image Gallery | How to build a Large Scale Forest | Foam rock carving and scenery | Layout Expansion