Building A Trestle Span

By John Kraker

TrestleRecently, I decided to add a new canyon to my N-scale layout. The plan was to build three separate trestle spans across my new canyon, after I build my new canyon of course!

This was my opportunity to create a steel sub-girder trestle 280 feet long and 75 feet tall, plus two main line trestle spans, 200 feet long and 50 feet tall! Not to mention the possibility of some deep-water scenery and maybe a chance to model a boat.

I purchased a supply of ‘Micro Engineering’ forty and eighty-foot steel sub girders and their “Tall Steel Viaduct” trestle kit. The sub-girders support the long sweeping curve as trains rumble over the canyon.

The model looks great and can be built to support trains for any curve, and length. I decided on an arrangement of forty and eighty foot girder spans. It would make an interesting focal point to what was, dull scenery. I know trains are fascinating to watch, even more so when they cross bridges and trestles or appear from and disappear into tunnels.

My N-scale layout, is a tabletop on steroids. It is 800 by 400 feet (2½’ x 5′) scale. I planned it as a display to run transition era freight trains, rather than operate as a railroad. I have two levels, ten switches and four trestles on the layout, not to mention eighteen power blocks and two throttles. The switches are all hand laid and make the layout possible.

I formed the scenery base with polyfoam and fiberglass screen, because of its strength, and lightweight. By design, this layout is subject to moving and whatever related stresses that may bring. The idea was to model detailed rock formations, and water scenes, rather than wooded, timber areas. Enough about my railroad, back to the curved trestles.

To start with, I made a tracing of the track to be spanned. I used freezer paper, shiny side down, and taped it to the track at each end of the span. Next, I traced the top of the rails to be supported by the trestle with a lead pencil and secured that tracing to a wood base with masking tape. This template will be used to shape the curve of the trestle.

I decided on the girder sequence and assembled the girders into modular trestles. When I was done, I had three forty foot spans complete with cross braces and two sets of eighty-foot spans without braces. Two of these eighty-foot girders are shortened to introduce a curve to the trestle. Remember the template is a top view and to build the trestle as you look down on it.

Eventually I wound up with three modules; two four leg, A-frame, steel support towers, and a module of five sub girders that follow the curve of the track. I finished cross bracing the span and painted the hard to reach places in the support towers before cementing them into a module. Once the trestle was completed, I finished painting and weathering details. Now I had to remove my canyon from rolling scenery. I say scenery, but it is basic. No color, everything is white resin cast plastic rocks or tan colored poly foam shell. At first glance, it looks like I modeled snow!

I laid out the canyon area with a felt tip pen, and proceeded to cut the line with a sharp razor knife. The polyfoam was hot glued to the bench work and can be removed easily with a sharp box knife. A characteristic of the polyfoam is how clean and easy it cuts with a sharp blade.

With the polyfoam removed, the bench work is exposed. I marked all the braces on the bench work supporting the road, and began the process of removing and designing the new bench work. In this case I was looking at a roadbed, wires, switch motors, bench work and three bare tracks spanning an open hole. I could no longer run trains!

I rebuilt the bench work, and formed the shape of the canyon with polyfoam. I added cast resin rocks in the canyon walls and floor to make it a rugged and dangerous place to be. When finished, these huge rocks will be surrounded by water, and concrete pyramid footings will support the steel towers on the rocks.

Now, with basic scenery complete, I needed to fit the trestle into the canyon. I assembled the support towers right out of the kit. The length of each individual tower support leg would need to be determined allowing it to land on a concrete footing and square up to support the trestle. This is easier than it sounds. I used a 3:1 mixture of sculptamold and lightweight hydrocal to cast eight rectangular blocks ¾” x ¾”x 1 ¾”.

When they cured, I used a bench mounted disc sander to shape them into a blunt pyramid. I drilled a 3/16″ cavity 1″ deep into the blunt end to accept the tower legs. This cavity allows for an adjustment while fitting the trestle into the canyon. Cut the bottom of the tower legs about ½” inch shorter than needed to reach the ground. Next, slip the tower leg into the cavity of the pyramid footing. After checking the fit to the rock, use a file to fit the pyramid base to the rock. I repeated the process with each tower leg.

I was ready to unitize the trestle. Using (extra thick 10-25 sec.) cyanoacrylate glue, I filled the pyramid footing cavity ¼” from the top. Only one footing block should be attached at a time. I positioned the trestle into the canyon; and used tape to secure it to the track that spans the canyon. When you are satisfied the trestle is in the correct position and level, verify the location of the footing and let the glue set up. Repeat this until all eight footings are attached to the support tower legs.

Finally, I used a hobby saw to remove the original track. Micro Engineering Bridge Flex track, was used to replace the standard track across the trestle. The ties are wider and spaced closer and look true to scale 1:160. The trestle kit, model #75-518, comes with guardrails, water barrels and platforms. I am sure this kit is available in HO scale. Use cyanoacrylate glue sparingly to attach the trestle to the bottom of the ties when you finally finish the scenery.

Just two more two hundred-foot spans and I can run trains!

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