Birchardville Railroad
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The Real Thing -- the Tunkhannock Viaduct

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The Real Thing -- the Tunkhannock Viaduct from the East

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I used the computer drawing of the arch printed full size as the basis for a stencil, which is attached Click to enlarge

to the Styrofoam blank with bank pins or foam nails

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before spraying to mark the cuts.

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In the spray box, airbrush over the stencil edges to identify cut lines. This is acrylic paint, in any old convenient color.

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One of the arches ready for drilling and cutting. I needed five, so I made six to have one for experimentation.

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Warning -- before machining extruded Styrofoam with woodworking tools, be sure you have a good way to keep the dust under control!

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Ready to drill the spandrel arch tops -- I used a 1/2 inch Forstner bit. Note the dust collection hose just to the left of the drill bit.

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Part way through the foam. If you pause, friction will melt the foam.

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Done with that hole. Ten more for this one, then five arches to go.

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After drilling the row of holes at the top of the spandrel arches Click to enlarge

Experiments -- near the upper end of the arch, I tried a rigid hot knife. It cut easily, but not on my lines. Near the lower end, I'd used my sabre saw.

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The sabre saw worked well, but made a whole lot of dust, away from the dust collector.

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I made a cutting platform from Styrofoam scraps held with clamps, holding a dust collector hose in place to suck up the dust as it was generated.

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Cutting the spandrel arches

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with a whole lot less dust loose in the shop!

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The pier pieces have slots machined in their sides to fit over the ends of the arches

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just like this.

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Dry fit the pieces -- if carefully cut, the really fit. Measure more than twice before cutting once!

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I glued this up with Loctite PL300 foam panel adhesive. Make sure things are straight before the glue sets up.

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All together -- there are two layers 1/8 inch thick balsa at the bottom of the piers, which forms the two (roughly) two foot steps at the bottom of the piers.

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Ready for paint, finally!

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Done!

Model Railroad Bridges and Viaducts

The Lackawanna Railroad's Tunkhannock Viaduct

Here's one link to a site that describes the prototype quite well:
The Lackawanna Railroad's Tunkhannock Creek Viaduct

For me, this was an experiment in scratch building. A number of years ago (December 2003), Model Railroader magazine had an article describing its author's building of a model of this massive viaduct in HO scale using extruded styrofoam as its main component. My model railroad that was in dream mode at the time needed that viaduct, as it is nearby (only 40 minutes by back roads). A bit of actual planning showed that I had a valley that was going to have to be crossed that would be (in model size, over five feet wide and a foot high.

I could not use the whole viaduct even in N scale, as the prototype is about 2400 feet long and 240 feet above the streambed! In N scale, that would be 15 feet long and 1.5 feet tall. Using selective compression, I modeled about half of the prototype -- five arches each about a foot in size. The original viaduct supported double tracks, but when the rail line was rebuilt, it became single tracked. My bridge supports a single track.

Single arches were designed using AutoSketch, a "lite" version of AutoCad. In the design, I picked some dimensions for ease of cutting. PDF copies of the drawings are available for download and/or printing:

The pictures to the left were taken during the construction, principally to show the how I designed and built the structure. The styrofoam board can make a huge mess when being cut -- so having a good dust collector in your shop might be an important first step. An alternative cutting approach would be to use a rigid hot wire cutting knife or a hot wire rigged like a scroll saw. Both of these would get the job done with a lot less styrofoam dust. In the pictures, you'll see an early attempt using my scroll saw, which became too messy even in test mode, and the cutting platform I rigged so the dust collector could do its work. This was quite satisfactory, and kept dust levels at a minimal level.

The last picture is of the viaduct structure as completed, but before the valley sides (or mountain sides) were roughed in. I had some PollyScale concrete and aged concrete paints which looked pretty good, but didn't like the price since I'd need a lot of this paint. I had tried the paint on some small pieces of foam board to check the colors. I then took the samples with me to my nearby (well, 25 miles is nearby here) OBB (Orange Big Box -- Home Depot) or BBB (Blue Big Box -- Lowe's -- I don't remember which, but they both have thousands of color chips in racks to help choose your paint color). After several minutes of searching, a young woman asked if she could help -- I explained I was looking for a paint to match this sample... and she said let me put your sample in the computer and we'll match it. So, I got a pint of matching paint. It took a number of coats to finish it -- mostly because I kept using Spackle and caulking to fill cut imperfections, then would re-paint the area.

The moral of this story is you can get pretty good color matches without paying for dozens of little bottles of paint. It is my intent to add streaks of "aged concrete" and "concrete" PollyScale over the structure as it is, but that will be only a small amount of paint.

By the way, a 14 year old granddaughters who hadn't seen the bridge model under construction saw it after painting and asked her dad if it was the bridge she sees almost every day. Hmmm.. must be good enough, but for you rivet counters, I only have 5 arches overall instead of 10 in the prototype. Also, my arches are not as tall as the prototype's.

Some credit where it is definitely due:

  1. The Model Railroader magazine article: December 2003
  2. An awesome book (privately published, without ISBN), TUNKHANNOCK The Great White Bridge at Nicholson, Pennsylvania, and the Lackawanna Railroad's Clarks Summit-Hallstead Cutoff New Century Edition, by William S. Young, Starrucca, Pennsylvania. Copyright (c) 2007, 74 pages including soft covers, with many photographs and drawings. (I own two copies of this book, btw.)

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Contact Info:
Name: Dan Janda
Email: birchardvilleobservatory@yahoo.com