Building A Trestle In 1/2" Scale

Building A Curved Trestle (Original article appeared in the OVAR Interchange Newsletter May'97)
The Denver, South Park & Pacific RR moved locations a couple of years ago for some very unusual reasons (especially for a railroad) the fore most was to take advantage of the topography which provided greater vertical variation / interest, something the previous location lacked. Thus, this terrain provided the opportunity for one truss bridge and two eight foot long trestles. And just to make things a little more interesting, both trestles were on 5 foot radius curves.
Step 1
We shall begin our construction with the bents.To start with, I drew up plans on my CAD package for the bents based on the Rio Grande Southern trestle design. This was printed in half inch scale and then these pages were glued to a sheet of plywood. Next some small blocks of wood were added to make this into a jig for the bents. Since I had to build 23 bents, I wanted to make this as easy as possible. Another jig was fabricated to fit on my 10" table saw to facilitate uniform cutting of the various bent wood pieces. 23 bents require a fair number of duplicate pieces. Titebond II glue was used to assemble the bents.Step 2
After the bents were fabricated, we moved onto the bridge deck.Before we launch into building the trestles, lets take a minute and understand some fundamentals about bridge construction. First and foremost, there is no such thing as a curved bridge. All bridges, single or multiple spans are just a series of straight bridges. Not only is it difficult to bend large steel plates (and wood) but the outer edge of a curved bridge is unsupported so when a train passes over the bridge the structure would twist sending the train into the valley below. Figure 1 illustrates the method used by railroads to bridge a curved track. Notice how the bridge spans are a series of straight sections, yet the curved track always remains between the straight side members.
These short bridges are nominally 15 feet long and sit on vertical uprights - bents to span a shallow depression. Wooden trestles, though very impressive to look at and quick to build, were a maintenance nightmare for the railroads and were often filled in or replaced with steel units as soon as possible.Of course now I'm going to say 'Do as I say, not as I do". Unlike my 1/2" scale wood structures who are taken in doors during the winter, the trestles will remain outside year round. My fear was that the winter weather and frost would wreck havoc with these delicate structures. So we took a couple of precautions to minimize this effect. First off, we chose to make the trestle one complete continuous structure. This is in complete contradiction of what we discussed above.
A full size cardboard template was made of the trestle and laid on a sheet of plywood. Next three 1/4"x3/4" cedar strip strips were glued together in this jig that resulted in a 3/4"x3/4" arc with a 5 foot radius. As illustrated in this photo, numerous clamps and sticks were required to hold the stringers on place while the glue dried. After the glue has set and the clamps removed, the stringers hold the curve exactly.
The stringers were then run through the thickness planner to smooth out the top & bottom edges.
Step 3
Next we install the ties.
Ties were glued between two such stringers in straight sections of 15 scale feet. When viewed from the top, it appeared as though the trestle was made of a series of 15' bridges even though the frame was curved. See Figure 2.
Next, the ties were glued between two such stringers in straight sections of 15 scale feet. When viewed from the top, it appeared as though the trestle was made of a series of 15' bridges even though the frame was curved.
Pieces of wood were used to space the ties evenly and lead weights to hold the ties until the glue has set.
Step 4
Rail is intalled. The ties were then sanded smooth with a belt sander (boy I love these large scale projects!) and the rail was pre bent and spiked in place. This involved drilling starter holes and sending the spikes home with a hammer and nail set.
Every 5th tie is spiked on both sides. The code 250 guard rails were installed after the code 332 rails were in place.
Step 5
Installing the bents. This stinger / tie assembly was then flipped over and the bents were then glued onto the bottom of the stringers at 15 ft intervals. Finally, the inter bent cross bracing was added along with handle railing and misc. timbers. Though the whole assembly is very strong, I felt we needed to go one step further - add a 2x8 sub frame.
Clamps are used to hold the diagonal braces in place until the glue dries.
Figure 3 and the enclosed photograph illustrates how this sub frame came together. The trestle bents were nailed to the 2x8's with a layer of landscape fabric (to minimize plant growth under the structure) in between. 
This whole assembly was 'dropped' into the abyss and carefully leveled before being back filled in. Crushed clay bricks were scattered under the trestle for appearance. Remember, the DSP&P had two trestles of similar size, so this whole process was repeated for the second unit!The cedar remains unfinished and after a year in the elements, is now turning a light grey colour. Both cedar and pressure treated lumber were chosen for their longevity in the Canadian climate. The trestles spent their first winter covered in plastic and it appears they have faired wonderfully.There are no mechanical fasteners in the trestles, instead a yellow carpenters type glue was used throughout the project that goes by the name of Titebond II. You can now find it most hardware stores as well as at Lee Valley. This glue is water proof and stand up well to outdoor use.Just a few additional statistics for you:
Materials consumed;
Here is the larger two story trestle in place on the railroad.

This Web page is written and maintained by Grant Knowles.
This page was last updated on Oct 11th, 1999.
This page has been accessed times since Oct 99.
URL: www.victorian.fortunecity.com/lion/829/gn3/articles/trestle.htm