The In-ko-pah Railroad


At one point on my layout, the track crosses a canyon in close proximity to the footpath on the upper level. In order to prevent damage caused by occasional bumps, I decided to use a cast concrete viaduct here rather than a trestle or bridge.

The viaduct was cast upside down in a wooden mold on my patio.

To build the mold, I started by cutting two 3/4” thick planks to match the curve of the track. The first plank was cut to the same width as the track, the second plank was cut about 2” wider. The narrower piece was glued on top of the wider one. This would shape the top of the bridge with a groove for the track to fit into.

The sides of the mold were cut from 1/8” luan plywood door skins. Because the bridge will be on a 4% grade, I had to cut the arches at a slight angle.

Pieces of illustration board were cut into a decorative pattern and glued onto the sides of the mold, The sides were then secured in place with screws.

I added 1” square strips of wood to the sides pieces, along the edges on the “legs” of the bridge. These would give me something to attach the inner arch pieces.

The inserts which formed the arches of the mold were made from sections of 8” diameter cardboard tube, glued to sheets of luan plywood.

All interior surfaces of the mold were sprayed with Pam, to act as a mold release.

I laid a couple pieces of 3/8” rebar into the bottom of the mold, using small scraps of concrete to raise them off the bottom. Then the arch inserts were clamped and screwed in place. 

I used a mix of high strength fast setting mortar, and vinyl concrete patcher. The vinyl patcher has a dark blue-gray tone, which mutes the white of the mortar to produce a color that approximates the look of aged concrete.

The concrete mix was poured into the mold, and rebar was inserted into the “legs” of the bridge.

After allowing 24 hours for the concrete to set, I removed the mold. I waited another day or two before attempting to move the bridge.

I moved it with the help of a neighbor. It weighed more than I expected -- I’d guess it was about 300 pounds. We had no trouble moving it horizontally, but getting it up the steps, and onto the layout, was difficult, and the bridge developed a crack. A few pieces of the rim forming the groove for the track also broke off, but I was able to glue this back in place..

Once the viaduct was in position, I cemented it in place with rocks and mortar, gradually building up the cliffs and canyon floor.

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