Trestles – RGS Style

Wooden Trestle Construction

on the Rio Grande Southern Railroad

by Bob Hyman

Part 2 - Model Trestles

There are two different approaches to building a model trestle. The first is to build a trestle to cross an existing opening in the layout scenery. This is similar to the task faced by the prototype bridge engineer. Each bent and panel must be designed to fit the existing terrain. The second approach is to build the trestle first, based on your personal desires, and then to build the layout scenery base up to the bottom of the trestle bents. Of the two approaches, the latter is my preferred method, although in either case, the basic construction sequence is the same.

Unlike the prototypes, we generally build model trestles from the top down. This means that the trestle deck is fabricated first, and then the individual bents are placed under it. Building the deck first allows the modeler to easily incorporate features such as compound curves, mixed curved and tangent segments, and varying grades into the finished trestle.

The Process

Trestle building is a process composed of the following steps:

Plans

The plans used for the trestle may be from an actual prototype structure or a proto-lanced design based on your own personal preferences and constraints. "Proto-lanced" means a freelanced design based on prototype practices. In either case, you need a scale-sized set of plans for each component, minor subassembly, and major subassembly in the trestle. If you do not have a set of plans to work from, they are easy to do on a personal computer.

One of the most puzzling terms a modeler runs into is "Degree of Curvature". If you are modeling a trestle on a 20-degree curve, just what does that mean? How do we translate that into a real radius for our model? Suppose that points A and B represent the ends of a hypothetical trestle on a 20-degree curve. To compute the actual prototype radius in feet, simply divide the number 50 by the Sine of one half of the degree of curvature. In our example, the prototype radius works out to about 288 feet (50 divided by the Sine of 10 degrees). All 20-degree prototype curves have a radius of 288 feet, no matter how far apart A and B actually are. To find the radius for your model trestle, simply divide the prototype radius by your scale. In S scale (3/16" to the foot, or 1 to 64), dividing 288 feet by 64 yields 4.5 feet. So, in S scale, a prototype 20-degree curve is about a 54 inch model radius. In F scale (15mm to the foot, or 1 to 20.32), dividing 288 feet by 20.32 yields a model radius of about 14 feet two inches.

This explanation of curvature may come as a bit of a shock to those of you trying to model in S scale with a 36 inch (or smaller) minimum radius. The sharpest curve on the RGS, at the Ophir Loop, was 24-degrees. This works out to slightly under 4 feet actual model radius (about 45 inches). The roofs of duckbill coaches actually touched going around this curve. I can't imagine a real train trying to negotiate a modeler's 36 inch radius curve. This would have represented slightly over a 30-degree prototype curve -- a physical impossibility for most equipment. Although you may have to "cheat" a bit on your curves, don't try it on a trestle. The visual distortion is amplified, and becomes quite noticeable to your viewers.

Jigs

I have found that jigs make most parts of trestle construction a whole lot simpler. A jig is a temporary fixture that allows you to easily build and assemble the various trestle components. There are two types of jigs -- fabrication jigs and assembly jigs. A fabrication jig is a tool that simplifies the making of a particular component. It allows you to make multiple copies of a component and guarantees that all parts made with a given jig will be identical and interchangeable. An assembly jig is a tool used in the trestle assembly process. It provides uniform spacing and positioning of the components into subassemblies, and of the subassemblies into the completed trestle. I typically make my fabrication and assembly jigs from acrylic (Plexiglas) or styrene, although some special-purpose jigs are made from wood. As a general rule, a jig is appropriate anytime you need to make more than one of anything. I often create a jig even if I only plan on making one of something. Jigs do not have to be pretty or neat -- no one will see them but you. They do have to be accurate. The finished product is only as good as the jigs it was built with.

Materials

Trestles involve massive amounts of scale-sized timbers. For the smaller scales (up to 1:64), I generally use commercially available basswood dimensional lumber. For the larger scales, I find it much more economical to cut my own timber from basswood, redwood or cedar on a table saw. Sheet metal is easy to replicate with heavy-duty aluminum foil. Rail is commercially available in all sizes. The 1:20.3 trestle sections prepared for this clinic feature Llagas Creek code 250 and code 215 rail (see sources at end of handout). Plastic or metal NBW castings are also available in many different sizes from multiple commercial sources.

The first task in material preparation is to add the appropriate wood grain to the timber components. Commercially available strip wood is too perfect and needs to be distressed. For the smaller scales, I usually dress each side of the timbers with a razor saw or wire brush to add the grain texture. In the larger scales, where I have cut my own timbers, the edges are usually okay just as they come from the table saw. Minor touch-up with a rasp or coarse file is sometimes required to remove traces of the saw blade cutting marks. Also, the pieces of aluminum foil used to represent metal sheathing need to be aged and weathered. I have had good luck using Radio Shack Circuit Board Etchant to remove the sheen from aluminum foil. Be sure to follow good safety precautions when using the etchant.

Raw untreated bridge timbers are typically various shades of whitish-yellow, light yellow-brown or reddish-pink when first cut. Various species of lumber appear differently when freshly cut. In old photos that show Howe truss spans in trestles, the color of the wood in the trusses is noticeably different from the color of the other trestle components. Most wood timbers weather to a common medium brown or orange-brown shade after a few short years of exposure to sunlight and water. Over a longer time span -- maybe thirty years or so -- this brownish color gradually turns darker, with grayish-black tones. Eventually -- after around fifty to seventy-five years -- the dark timbers will weather to a lighter silvery-gray color. At higher altitudes, perhaps above 9,000 feet or so, the orange-reddish hues will become more predominant over time. This probably has something to do with the strength of the ultraviolet radiation.

The raw timbers for the 1:20.3 scale trestle sections prepared for this clinic were cut from 3/4" x 4" x 6 foot cedar fence boards using a standard (10-inch) table saw. The cedar fence boards are available in most home and garden centers (Home Depot, Lowe's, etc.) for about $1.19 each. The timbers were dipped into a 50/50 mixture of Minwax brand, water-based Colonial Pine wood stain and water, and then wiped dry with a rag. This tones down natural orange hue of the cedar without completely hiding the grain and texture. I think it is a good representation of coloring for typical 20 to 30 year-old bridge timbers (perfect for 30's and 40's era modelers).

Bridge timbers were untreated on the RGS bridges, so no creosote effects are needed. Rust stains are appropriate where metal parts are attached. Just remember that most trestles were brown or brownish-orange shades during the years of operation. Don't be tempted to color your trestle timbers based on what the wooden remains look like today!

Minor Subassemblies

A minor subassembly is something made from two or more individual components. It is first assembled by itself, and then later added to other subassemblies to create a major subassembly. A good example of a minor subassembly is the first story of a bent. This minor subassembly may be common to several different bents. Once we have the design for one, we can build as many copies as we need for all the bents.

Major Subassemblies

A major subassembly is something made from two or more individual minor subassemblies. Like the minor subassembly, it is first assembled by itself, and then later added to other major subassemblies to create the trestle. The major subassemblies of a trestle include the deck and the individual bents.

Assembly

The major subassemblies are combined to form the trestle. I usually start by temporarily securing the deck subassembly to the deck jig; then I place the deck subassembly into position across the opening to be bridged. I then add the individual bents to the deck, starting at one end and working across the bridge. If the trestle deck is level (not likely), the bents are perpendicular to the deck. If the trestle is on a grade, then the top of each bent must be slightly angled so that the bent is vertical to the ground. I accomplish this with the aid of a bent installation jig.

Details

Some components are best left out from the subassembly process. These are things like bent footings or foundations, girts, wall bracing, number boards, platforms, and galvanized metal pieces. It is usually easier to add these details to the trestle after all of the other major subassemblies have been assembled.

Bridge number boards were located at the north (Ridgway) end of most trestles. These were generally on metal signs, nailed to each side of a piece of 4" x 8" timber, which was inserted between the fifth and sixth ties on the bridge. The timber extended out horizontally about six feet beyond the guard timber on the engineer's side of the trestle deck. The metal sign was painted black with six-inch high, white block letters, e.g., 46A.

Platforms, used as refuges and for fire barrels, were located on some of the longer trestles. The exact placement and number of platforms varied over time, so check photographs for the era being modeled. The platforms were generally supported on two pieces of 4" x 8" timbers, inserted between the ties, similarly to the bridge number boards. The platform floors and railings were fabricated from whatever lumber was available, typically 2" thick and 6", 8", 10" or 12" widths.

Girt arrangement and timber size varied between trestles. Some girts were notched to fit over the sills; others were left un-notched and laid flat on the sills. Some girts met end-to-end at the midpoint of the sill; others were lapped side-by-side, with each extending the full width of the sill. Girts were usually parallel to the trestle stringers, but occasionally were angled to join sills at different heights. Refer to photographs of specific trestles for details.

Concrete foundations were poured into wooden forms. When the form board were removed, the lines between the individual boards and the imprint of the wood grain remained in the concrete. These are easy details to model, simply by casting the foundations from plaster in strip wood forms. Prototype foundations had metal straps embedded into the concrete that anchored the timber mud sill to the foundation. These straps were typically 1/4" x 2" flat steel and encircled the mud sill at each end and at the center.

The various prototype timber components were joined together with a variety of metal fasteners including bridge nails, lag screws, drift pins, and square-head bolt with nuts and washers. Refer to photographs and drawings of specific trestles for details. On a model, when in doubt -- add a tiny NBW casting where the nails or lag screws are indicated. If anyone questions a NBW in a particular location, simply explain that it is a bridge nail or lag screw that has started to back out due to vibration. This is very prototypical. In fact, some photographs of the high line bridges show the lag screws that anchored the ties to the stringers backed out several inches from the surface of the ties!

Tools and Adhesives

My method of trestle-building revolves around the use of a table saw. In larger scales, I use a standard 10-inch table saw to rough-cut the raw bridge timbers from boards. I prefer to use a 4-inch table saw to rough-cut timbers for smaller scales. In all scales, I do the final trimming and end-cuts with the 4-inch saw. Both table saws are equipped with carbide-tipped blades.

A smooth, flat work surface is critical for proper alignment of bent and deck components. Usually, the components are assembled on a jig that provides the necessary surface. In some cases, however, a smooth table-top is needed -- particularly for the taller bents in larger scales.

Clamps are handy for holding the various pieces in alignment until the glue dries. I use a variety of C-clamps, bar clamps, and even rubber bands. Weights are useful to apply pressure to flat surfaces that do not lend themselves to clamping. A good example is when gluing cross bracing to bents.

Plexiglas jigs are assembled with Elmer's Polyurethane Adhesive. This is stronger than wood glue, and will bond just about anything. Keep a bottle of acetone handy for cleanup. It works just like ACC "super-glue", but gives you quite a bit of time to properly align the parts before it sets.

Paper plans are attached to the Plexiglas jigs with generic, spray adhesive (3-M, Duco, etc.). I spray the back of the plan lightly, then align the bottom edge, and press down the plan with a soft cloth, working from bottom to top and center to sides. Wooden blocks are glued to the paper plan with Elmer's Carpenters' Wood Glue. Plain old Elmer's white glue would probably work just as well. When I am through using the jig, I peel the paper plan from the jig, and wipe off the glue residue with a paper towel dampened with acetone or turpentine.

Holes for NBW casting are easily drilled using a Dremel tool mounted in a drill press. You can do this by hand with a pin vice, but in the case of a large trestle, it is very time consuming to hand-drill thousands of holes. Fortunately, most trestles are nailed together, rather than bolted. Even so, you still need to add the nail heads!

I use Elmer's Carpenters' Wood glue for attaching all trestle components together -- the wood, the sheet metal, even the NBW castings and concrete (plaster) foundations. If your trestle is going to go outdoors, you may want to consider a true water-proof type of glue. I would also recommend using finish nails through the caps and sills into the posts, just like the prototype drift pins, for outdoor use.

When the glue dries, it will invariably stick to the paper plan under the joints. Simply slide a single-edge razor blade between the wood and the paper to separate them. The tiny bits of paper that stick to the wood can be easily scraped off after the bent is removed from the jig. Or, if you prefer, you can cut out openings in the plan at the joint lines with a hobby knife after the plan is attached to the jig. The glue will still seep out of the joint, but it won't stick to the Plexiglas. With practice, you will know just how much glue to apply to minimize the seeping. Keep a damp rag handy as you glue the bents together. Wipe off any excess glue that seeps from the from the exposed areas as soon as possible to minimize discoloring and to simplify final clean-up.

Making and Using Jigs

The type and quantity of jigs necessary depends on several factors: the complexity of the trestle itself, the degree of accuracy desired, and the type of tools used to fabricate the parts. The scale of the model does not affect the need for jigs.

I fabricate model trestles using a small table saw. My particular variety is a Dremel 4" Table Saw. Most of my fabrication jigs are made to fit onto the saw. I use the guide slots on the table to position the jigs. I start by cutting a quantity of .093 inch thick Plexiglas strips to fit into the guide slots. I then attach these strips to Plexiglas sheets with Elmer's Polyurethane glue so that the strip rides in the guide slot with the edge of the sheet flush against the saw blade. The Plexiglas sheets are available in various sizes (8" x 10", 11" x 14", or larger).

First, print out (or draw) the plans in the desired scale. If the plan is larger than a single sheet of paper, print it on multiple sheets, and align the sheets using registration marks on the plans. Assemble the multi-sheet plans together with a child's "Glue-stick", available in any craft store. Attach the plan to the Plexiglas sheet with spray adhesive. Align the bottom edge of the plan cut line with the edge of the Plexiglas sheet that touches the saw blade.

Glue small pieces of Plexiglas strip (or strip wood) to the plan along the component lines. It is better to use small pieces spread out along the component line rather than one long continuous strip.

The most important jig that you can make is the Batter Jig. This jig has positions for cutting the end of a timber at one of four angles: perpendicular, 1 in 12, 2 in 12, or 3 in 12. This single jig will be used to cut nearly every timber used in the trestle. I also make a stop block to fit onto the perpendicular cut position of the jig. This allows me to make any number of identical length timbers.

Story jigs are made for each story of every bent in the trestle. The process for making a bent story is as follows:

After all stories for a bent are fabricated, they are assembled together and the sway bracing is added. I usually design my 2^nd story jigs so that the upper 1^st story can be glued to the top sill. Similarly, the 3^rd (and subsequent) story jig holds the previously prepared upper stories. In these cases, the jig serves dual purposes; it is both a fabrication jig and an assembly jig.

Another valuable jig is the Guard Timber Notch Jig. As explained earlier in the prototype trestle section of this handout, the guard timbers were typically notched to fit over the ties. This is a difficult and time consuming detail to model, but the results are well worth the effort. The task is greatly simplified by using the Notch Jig. The notch Jig is a special type of stop block made to fit on the left side of the table. The saw blade is adjusted to a 2" scale cutting depth. A 5" x 8" timber is paced on the perpendicular line of the batter jig, on the right side of the table. Starting at the bottom of the Notch Jig, the guard timber is repeatedly run through the saw. After each pass, the timber is moved to the next higher position on the Notch Jig. The jig allows the modeler to quickly make a series of perfectly spaced, 2" deep cuts across the timber at 8" and 4" intervals. After all of the interval cuts are made, the Notch Jig is removed from the table. The area between the 8" cuts is then removed by making a series of passes across the saw blade. With each pass, a portion of wood equal to the width of the blade (about 3/64") is removed. The notched guard timber is further sized to create the lap joints as explained in the prototype section.

Another important jig is the bent positioning jig. Unless the trestle is perfectly level (highly unlikely on the RGS), the bents will not be perpendicular to the deck. The bents are always vertical. Since the deck is on a grade, the angle between the bents and the deck is not ninety degrees. To calculate the correct angle, simple take the inverse sine of the grade. For example, a three percent grade is .03 (3 divided by 100). This represents a three-foot rise in a 100-foot length. The inverse sine of .03 is 1.72. This means that the angle between the deck and an imaginary level line is 1.72 degrees. It also means that the angle between the bents and the deck is 90 – 1.72 degrees, or 88.28 degrees. The prototype trestles adjusted for this angle by using shims or by shaping the bottoms of the stringers where they touched the bent cap. For model trestles, it is usually easier to slightly angle the tops of the bent caps. The bent positioning jig is simply a block of wood cut to the correct angle. Notches in the side of the block allow it to fit around the sway bracing and against the bent post. The angled edge of the jig fits against the bottom of the deck.

References and Sources

Sample High Line Trestle Photos and Drawings

Here are some of the sample F Scale (1:20.3) trestle sections used during the clinic.  These samples are from Bridges 46-C, 46-D and 57-B. (Click on images for larger versions.)

Here are some photos taken on my Sn3 diorama of Bridge 45-A.  (Click on images for larger versions.)