Scenery Construction for the Trestle
( A work in progress)
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The new Fascia board added to the framing. I made temporary track supports to allow running while the trestle is removed. 
Screening added in the Trestle Valley. Plaster cloth will follow and then rock castings will added.
These are the footings for the trestle. Each is a custom fit so the trestle just slides in and has support..
Here the trestle is set in for a test fit after the screening has been installed.
First plaster cloth strips.
This plaster cloth hardens better than I expected and dries quickly.
The gully plaster cloth finished.
Lots of sheer rock walls to add rock castings to.
Canyon floor under the trestle has rock castings glued down like tiles. The molding plaster is tinted with mortar tint. I use Red, Yellow, Brown, Dark Brown and Black tint varying the mix a bit with each batch. A little tint goes a long way. Only use about a half teaspoon of each color in a one quart batch for casting rocks.  I mix all the colors but the black in the plaster before adding the water. I add the Black after the plaster batch is mixed with the water, stirring so the black is streaking the mix. This gives the rocks a better color texture.
Tinted Plaster is spread like grout between the rock castings. I only use a half cup of grout at a time due to the setup time. Brush the area you are grouting with a water before grouting  to keep the plaster grout from setting up too quickly.  Trowel on the plaster grout with an artist pallet knife or Popsicle stick.  Use a wet paint brush to work in and finish the grouting. By tinting the plaster rocks and grouting, painting of the final colors is minimized. 
The vertical areas are covered with pieces of broken rock casting hot glued to the wall. It is a jig saw puzzle for sure.
West Canyon Wall..

Taking shape now. It will be grouted also. The rock castings are cast only partially filling the molds so to be slightly hollow to allow easier cutting and fitting over an uneven base. 
East Canyon Wall
Rocks all glued in place with trestle in place for test fit.
Grouting between the rocks to blend together is next
East Wall West Wall