This shows the various stages of the construction of a mountain...

After laying the track so that we know exactly where the track would be (it is never exactly where the designer drew it to the mm) we constructed 'wire-frame' mountains out of K'NEX to get a feel for what it would look like. Working in 3D helps visualize what is to be built.

Then the roof of the tunnel going through the mountain is built and painted black, tunnel portals are cut and tested in place...

Note the blocks on the roof to hold copper wires to guide pantographs to the catenary...

The hillside that spans the access hole below is covered with steel mesh (6mm square) and stapled down to hold it in place.
It is then covered with cloth so that the plaster does not drip through.

The form of the mountain is now shaped using Styrofoam sheets that are glued down with wood glue.
Masking tape has been placed over the tunnel portals to prevent plaster from getting onto them in the next step.

Now covered with dry-wall plaster
I find I need to do two layers of plaster.

Places that are to be rock face are left lumpy at the plaster stage and then painted. If you place thicker layers of plaster on these areas it also cracks during drying and they work well for rock faces.
Small shape detail such as the hiking path are then scraped or carved out of the plaster

Next step is grass... I coat the surface with wood glue, then brush on sawdust that has been colored with green paint. When the glue has dried I bush off the unstuck grass.