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I cut 1/4x1/4 stripwood on a ten inch tablesaw from a piece of pine 1x4.
.060 thick styrene is used for the outer sheathing. I purchase styrene in large sheets from a plastic supply house.
Other materials can be substituted but I like to cut wood and use styrene for flat stock.

Scribing Styrene
Finding the right scriber for styrene has been a long search. I discovered this by accident. I use a Utility or Box Cutter blade. The blade is ground to remove the "roman nose" at the tip. Pulled along the scribe line it produces the perfect continuous chip. Little clean up is required. It can be sharpened repeatedly.
Making the Sheathing
The sheathing pieces are cut from .060 styrene as per the plan. 100 grit sandpaper is used to take all the finish off the styrene. The initial graining is made with 50 Grit sandpaper. This is done before scribing.
After scribing take a sharp utility knife blade and cut through the styrene. This makes separate board ends and marks the boards for the other side.
Take a hobby knife with #11 blade and roll around the cuts opening up the knife cuts.
Using the marks scribe lines on the other side.
More texture can be added to the styrene with a hobby knife. Too bad all this neat texture was covered when the portal was installed. Additional sanding with the 50 grit paper will add more texture.
After adding sandpaper texture the scribed grooves are best cleaned out with the backside of a #11 hobby blade. Just run it down the groove, it will have a nice curly chip.
Making the Timbering
Timber tunnels like this were put together with a mortise and tenon setup like a mine set. They lacked the lower traverse girt member.
I used a miter box screwed to a 1x4 with a stop block clamped on as a stop. I use a fine razor saw to cut the timber. This allows the uprights or posts which are between the caps to be the same length.
A small plastic caliper is used to make sure the posts are the same length.
I prefer scrapping with a knife blade to remove fuzz from the lumber. It works better than sanding.
Posts are sanded on a North West Shortline "Sander" to the same length.
Putting it Together
Staining the Plastic and Wood
I use a rather un-orthodox method of handling wood and styrene into weathered wood. I white primer the materials and stain the primer.
Any white flat white primer will do. Kilz2 is being used here it is diluted 75-25% Kilz to water.
Dye-na-flow fabric paint is used as stain. It is black with no other tones. I have went away from India Ink.
For the inner patina brown wood inside the tunnel Sepia drawing ink is used to stain the primer.
Click here for more on weathering wood and styrene
Since I was going to need the tunnel assembled to retrofit the whole in the mountain I primed the assembly with Kilz2 diluted slightly with water. Doing a new tunnel I would color the sides and top separately. Then assemble the sides on the layout and apply the top to them.
Apply black Dye-na-flow to the primer directly from the bottle. After finishing waterproof them by spraying with dull lacquer. I use Floquil Dull Clear.
This is the color we are trying to acheive a silvery gray.
In the interior of the tunnel it will have a dark brown patina. I use the Sepia drawing ink to acheive that color. Determine where the sunlight hits the tunnel and use sepia from there back. If the ink is applied to thick it will dry glossy.
Assembling the Tunnel
The girts, the top timbers, are glued to the sheathing with hobby glue space according to the plan
The angled caps are assembled on the plan. A piece of wax paper is placed over the plan to prevent it from sticking.
Mark the location of the posts, the vertical timbers, using the top. This assures they will line up.
With a square draw vertical lines and glue the posts to the sides. Use a stop to assure alignment along the edge of the side.
Place a weight on the side and glue the caps to the posts.
Click here for Dye-na-flow on the net
As stated above if this is a new installation just glue the sides to the layout and the top to the base and build the mountain over the tunnel. Mine was a retro.
I actually believed I could glue the whole thing together and it would stay together during painting and putting on the layout. D-OH!!!.
Since things are only mistakes if they can't be corrected, I concocted the idea of having more posts,  girts and caps to appear that the tunnel had been later reinforced.
The extra timbers made the tunnel workable and look pretty good, maybe not 100% prototypical.
I use these heads from cheap bargain store adjustable squares to make sure the assembly is square.

Click here to plant the tunnel on the layout
The tunnel once assembled was primered and stained.