Threstle Building

I don't know that there is anything special about the way I build trestles but building curved ones does require a different technique so I'll show how I did mine. I started with my bare track building bench and taped a piece of brown craft paper over it.

Next I marked the center line of the curve. This was done using a home made trammel that I made early on when I was cookie cutting PT plywood for road bed. It still comes in handy for laying out curves even on the layout. It's just a very straight 2x2 ten feet long. At the end where the carpenters pencil is in this photo is a universal mount where I can attach a Roto-zip tool. The pivot point is adjustable with center line spacing at 4 1/2', 5', 6' and 8'. The series of five holes seen just ahead of the center point are repeated for each of the basic radius settings. The inner tow establish the rail positions and the outer two were used when I was laying out roadbed to mark the outer edge of the ties. It's not necessary to have anything this elaborate, one only needs a stick with a hole for a pencil at one end and a nail in the other set apart at the radius you want. The trammel is set on a tripod stand that was work light stand. Once the center line was marked, marks that would locate the bents were marked starting at one end and marked at 6" intervals. The edge of the trammel was then used to create a line that was parallel to the radius.

The trestle bents when they were assembled were marked with a center line both at the top and bottom. The bents were then placed on paper where marked and were tacked in place using a pin nailer as shown below. Once that was done I started adding the joists connecting the bents at the top. It was necessary to determine their lengths first. The ones on the inside of the curve are a bit shorter than the center line spacing and the ones on the outside a bit longer. Technically the ends should be cut at an angle but I found that at a five foot radius the angle was so slight and to be unnecessary as there was plenty of overlap onto the bents and the slight gap on one side wasn't that noticeable. The joists were spaced to fall under the rail and centered on the bents. They were installed with both pin nails and Titebond III glue.

Once all of the bents and joists were installed The cross bracing and lower level stringers were installed. Again the length of the cross braces were determined by a bit of trial and error then once the correct length was found multiples were cut and installed. On these I attach them using 19 gauge x 1/2" finish nails. I drill slightly under size holes for these then apply glue and push the nails home with needle nose pliers that have a slot cut in them to help hold them straight. I leave the heads stand proud of the surface which gives a faux bolt head appearance. all of the wood used in this trestle was milled from standard pressure treated (non ground contact rated) wood. The key to this is to buy the stock well in advance taking care to select pieces with finer grain, the least amount knots and preferably as straight as possible. Once home clamp it down to a flat surface and allow it to dry thoroughly, turning occasionally. Some pieces in spite of you efforts will still warp and bow a bit but most will be usable. The stringers were made as long as possible then threaded through the bents and bent and fastened as I go. The photo below shows the bracing and stringers done for one side. The Bents are made from 1/2" x 1/2" stock and the cross bracing and stringers made from 1/4" x 1/16" stock. The 1/2" stock is milled on a table saw and the 1/16" stock is ripped on a band saw.

The next photo shows the other side with all the bracing and stringers in place. Note that the design is simplified from the real world. A prototype trestle would have the cross bracing with the diagonals on opposite sides of the legs. That makes fastening much more difficult and since the thin wood is flexible I mount both braces on one side and just bend them past each other. From a normal viewing distance this difference isn't that noticeable. At this point the trestle is ready for stain. I know some folks like to stain first to eliminate places that won't stain due to glue but I trust the glue to make a better bond without stain on it. The staining could be done by hand but it would be very time consuming so I roll my track bench outside and apply using a paint sprayer. The 50-50 mix of stain and linseed oil tends to take a while to dry so I leave it outside as long as possible and usually try to do it on a sunny day to speed drying.

The next photo shows the trestle after staining. Also on the bench, besides my sleeping inspector, are the doubled bents that will support the bridge that will be attached to the end of the trestle and a stack of bridge ties. These are Trex and unlike the ones I use for normal track work are ripped to 1/2" x 1/2" since they are totally visible when installed.

The next photo shows the ties being installed. I have terrible luck whenever I try to do something like this by eyeball. No matter how I try they usually end up drifting off from the proper angle so I came up with the jig shown below. Its just two 6' long pieces of strip wood fastened together at one end with a carriage bolt and wingnut. Near the opposite ends there is a mark at the 5' center line. The first tie is attached at one end then one side of the jig is clamped to the trestle joists. The wingnut is adjusted just tight enough for a friction fit and the loose end is supported but not attached to the tripod I used for the trammel earlier. I space the ties using another tie and fasten one end of the tie then use the jig to set the correct angle before fastening the other end. The ties are fastened to the joists using 18 gauge 1" stainless steel finish nails using an air nailer. Since the joists are spaced just below the rail center line the nails end up below the rail and invisible.

I didn't take a photo after installing the rails while still on the bench. It's shown below when I moved it outside to check the fit with the bridge it attaches to. The guard rails for the narrow gauge still need to be installed and that was done back on the bench. To remove the trestle from the bench I just start by prying up one edge of the bent bases where they are pin nailed and once one side is loose it can be pulled off leaving the pins in the bench which I will remove with a pair of pliers.