Tunnel Portal #2, In Detail

After a couple of weeks off due to some travel, I'm building the portal for the other end of my tunnel.

This is the finished portal. Much better than the first one, I think. There's just a little touch up to do to the black paint inside, where I had to trim the foam to fit the tunnel liner.

The design is based on pictures of real timber tunnel portals, especially the Goat Canyon tunnel on the San Diego & Arizona Eastern; drawings of a Western Pacific portal that appeared in the May 1969 issue of Model Railroader; and to be honest, pictures of other people's models.

The Model Railroader drawings show 12 x 12 framing and 4 x 12 planks, so I started by staining several lengths of 1/16 x 1/4 and 1/4 x 1/4 basswood. I used a wash of grey craft paint, followed by dry brushing with black.

I laid out the face of the portal on cardstock from a manilla file folder. The Model Railroader plans show an opening 17 feet wide across the inside face of the timbers, by 21 high. That's standard gauge. NMRA recommends 4-1/2 high by 3 inches wide for O scale narrow gauge. Because I'm only using small equipment on this layout, I went with 4 inches high, but because it's on a curve I went 3-1/4 wide. The portal face extends 1-1/4 inch on each side and the top. I cut a couple of test portals and ran various equipment through them to be sure they'd be large enough.

I cut the cardstock drawing out and used it as a template to cut a piece of 1 inch EPS foam to the same size. Then to allow for the tunnel liners, I trimmed an extra 5/16 inch from each side and the top.

On my first portal, I glued the stripwood to the cardstock, then glued that to the foam. This time, I only used a cardstock backing for the liners. The face timbers I glued directly to the foam. I did the liner roof first; glued it in place, making sure it was level, then added the liner sides.

Side assembly, on a cardstock backing.

Roof and sides glued to the foam core.

Notice I left a 1/4 inch gap at the bottom, where the sills will be added.

Timbers were glued to to the face.

Timbers in the process of being glued on.

Then framing: top cap first, then outside verticals, then the horizonal frame over the door.

Next I added the sill timbers. The MR drawings have them extending out 15 feet. Mine are only 10 because that's about all that will fit at the site. After that, I cut the other two verticals to fit.

Two short vertical filler pieces above the opening, diagonal braces in the top corners of the opening (three on each side), and the front braces. The drawing shows two diagonals on each side (the ones extending forward from the portal). I originally only included one on each side, but after looking at some more pictures, I added another after this photo was taken.

Test fit on the layout: I'm going to need to relocate the engine house track slightly to clear the front brace on the left, and build a retaining wall on that side where the hillside is nearly vertical.

A couple of days later, I finished it up. 

I built a short section of interior liner. The sides are built by gluing stripwood to a cardstock backing, then adding framing. Since it's on a curve, one side is shorter than the other -- the wall on the inside of the curve is 1-1/2 inch wide, and outside wall is 2 inches (the geometry works out; it's a tight curve). If I had wanted to, I could have made more sections and extended the liner farther into the tunnel, but based on the other portal, this seemed like enough. The tunnel is fully enclosed and the interior is painted flat black, so the lack of a liner further in is not apparent; anyway, the interior walls are not visible past the first couple of inches, from any normal viewing angle.

I added a cardstock roof. It won't be visible, but adds some strength to the structure.

Finally, I added a few nut-bolt-washer castings, touched up the cut ends with dilute grimy black paint, and did a little light weathering.

You may notice in this low-angle view that I added another frame to the liner, and diagonal braces.

So, that's that. There are still one or two little flaws, but overall this one is a fair bit better than my first effort. Now I need to relocate the enginehouse track slightly, because it interferes with the left front brace. Then I can install it. Wing walls or other retaining walls still to come once this is in place.

Update, Jan. 24: I built the wing wall, touched up the tunnel interior, and ballasted track in and around the tunnel entrance. I also mixed up some of the tan grout and applied a thin coat over the carved foam creek bed in the corner. Here's what it looks like as I wait for all of that to dry.