Trestles were once, and still are, common across North America. In early 1830, America had a total of 23 miles of track. However, by 1890 America's railroads had increased dramatically to 163,597 miles according to a page on the Central Pacific Railroad Photographic History Museum.
In order for this expansion to happen, many rivers and lakes needed to be crossed and many valleys, canyons, and gulches needed to be bridged, but bridges were expensive. However, one of the cheapest types of bridges was the timber trestle, and so that's what railroads used. Trestles had the advantage of being built in manageable sections from lumber that could be felled locally, or brought in by rail from forests hundreds of miles away. Timber trestles also used a minimum amount of iron, mostly seen in the bolts, nails, and staples used to secure the joints.
Trestles are constructed by driving vertical columns of wood into the ground and stretching to track level in an "A" frame shape. The number of and thickness of these vertical lumber columns was determined by local wood available and the weight of railroad equipment. Heavier equipment required a stout trestle whereas little locomotives and cars could work with a much cheaper trestle. Sometimes the trestle was temporary, other times the trestle was permanent. Regardless, the trestle was the answer Railroads needed to cross the continent and build branch lines in remote terrain. One little corner of the country where trestles are apparent is in the Black Hills of South Dakota. Today, trestles can be found on the Black Hills Central (1880 Train) line between Hill City and Keystone as well as the Mickelson Trail between Deadwood and Edgemont. Other remains of trestles can be found in the old mining districts, the Badlands, and on former roadbeds such as the Rapid City Black Hills and Western between Rapid City and the ghost town of Mystic.
To model railroaders, trestle building is a skill that can be quite useful. Instead of relying on the limited kits available to build such bridges, exact replicas or custom designs can be made to fit a particular valley on a model railroad. But before a trestle can be built, it helps to know how trestles are built. The following photos should provide excellent photo reference for trestles that would be seen on CBQ lines throughout the Midwest. The following subject was found between Hill City and Keystone on the Black Hills Central line in the Black Hills. Because this is a tourist line on a former branch of the CBQ, these trestles are built to a lighter standard than seen on mainlines.
In Figure 1 we get an overall shot of a typical railroad trestle. Most trestles crossed small ditches and other openings. In rocky terrain it would have been cheaper to build small bridges than spend time filling in these depressions, and then re-filling them every time a flood occurred. The Black Hills particularly had many floods with steep, narrow valleys and many heavy rains from prairie thunderstorms. This particular trestle is around 70 feet long and 12 foot high. Six bents support the structure and are sturdy enough to handle the weight of a Weyerhaeuser 2-6-6-2 Tank locomotive.
In Figure 2 we have a close-up view of the bridge, showing the various parts. The first part to look at is the superstructure. Like many trestles, this one is an open design, meaning that rails are secured directly to the structure. The superstructure, or trestle deck, is made up of rail, railroad crossties, and stringers. Some trestles, many of them steel, have a closed superstructure that lets the track be supported by ballast. 8x8 timbers separated 6-8 inches serve as railroad ties on this bridge. The small boards running along the top helped to keep the ties evenly spaced and are called runners. Obviously, they are not load bearing, unlike stringers.
Supporting the railroad ties and the rail are stringer boards. These boards are at least 12 inches thick and about 6 inches wide. They are laid on end rather than flat to increase their relative strength. The number and size of these stringers would vary depending on the available lumber and the weight of the trains which would run on the line, therefore the stringers could vary quite a bit in size. On this particular trestle there are 2 stringers that are composed of 2-3 laminated boards per stringer. The boards, when paired together, provide incredible strength. The joints could also be staggered, meaning that there is no complete joint in the stringer for the entire length of the trestle.
In Figure 3 we get a good view of the sub-structure of the trestle. The substructure of most wooden trestles consists of a series of bents. These particular bents are formed from 5 vertical posts of lumber. One, in the center, is perfectly vertical. The others splay out, providing a stable "A" frame shape to support the superstructure. All the posts seem to be around 10 inches in diameter. Capping off the vertical columns are 10x10 cross members known as caps. Similar to bridge supports on modern highways, these pieces of lumber evenly distribute the weight across the five columns, thus ensuring that none bears too much weight. To stabilize the columns laterally, they are bound with diagonal braces, one on each side, in an "X" shape. These are aptly named "sway braces". Between two of the bents was another "X" for further support. Smaller bents would not need these lateral braces as their small stature didn't allow for much flexibility.
In Figure 4 we have a better exposed image that allows us to clearly see some of the hardware used to build a trestle. The stringers are bolted together in a laminated manner. As stated, this practice increases the strength of the stringers dramatically. Usually with lumber this heavy, there's not much movement to worry about. Regardless, a precaution is made with these large straps which are bolted into the stringer, and then bolted into the cap. Of course, all the bracing was bolted in as well. Bolts were used over screws due to their greater strength and longevity.
In Figure 5 we get an excellent view of a railroad trestle bent. One thing to note is that there are no sills on this trestle. A sill is a piece of lumber running horizontal across the bottom, or middle of the bent in a similar fashion to the cap. It appears that with this trestle the posts were driven directly into the ground. Often large trestles would require their own pilings to secure the bents in place. Sills also gave a place for horizontal and diagonal braces to be anchored.
Figure 6 shows one of the shorter bents. These would be used on the hillsides below the trestle and were therefore shorter than bents found in the middle of the span. The stout proportions mean that these bents require less bracing than the taller bents. Underneath the trestle it is typical to have debris. This rock debris would be made up of railroad ballast that has fallen down the slope, rocks from hill grading prior to bridge construction, or rock intentionally put in place to aid water drainage around the trestle, not to mention deterring plant life from taking rood directly under the bridge.
And finally we have Figure 7 which provides us with a detailed look at a bridge abutment. Abutments are used on each end of the bridge to transition from bridge to ballasted track. On simple and small trestles, the abutment would consist of a very short trestle bent, as seen behind the weeds in the foreground, and supported by a wooden retaining wall to keep the ballast in place. Other types of abutments would include rock and mortar, cement, or wood cribbing.
As a re-cap, there is the terminology with definitions:
In order for this expansion to happen, many rivers and lakes needed to be crossed and many valleys, canyons, and gulches needed to be bridged, but bridges were expensive. However, one of the cheapest types of bridges was the timber trestle, and so that's what railroads used. Trestles had the advantage of being built in manageable sections from lumber that could be felled locally, or brought in by rail from forests hundreds of miles away. Timber trestles also used a minimum amount of iron, mostly seen in the bolts, nails, and staples used to secure the joints.
Trestles are constructed by driving vertical columns of wood into the ground and stretching to track level in an "A" frame shape. The number of and thickness of these vertical lumber columns was determined by local wood available and the weight of railroad equipment. Heavier equipment required a stout trestle whereas little locomotives and cars could work with a much cheaper trestle. Sometimes the trestle was temporary, other times the trestle was permanent. Regardless, the trestle was the answer Railroads needed to cross the continent and build branch lines in remote terrain. One little corner of the country where trestles are apparent is in the Black Hills of South Dakota. Today, trestles can be found on the Black Hills Central (1880 Train) line between Hill City and Keystone as well as the Mickelson Trail between Deadwood and Edgemont. Other remains of trestles can be found in the old mining districts, the Badlands, and on former roadbeds such as the Rapid City Black Hills and Western between Rapid City and the ghost town of Mystic.
To model railroaders, trestle building is a skill that can be quite useful. Instead of relying on the limited kits available to build such bridges, exact replicas or custom designs can be made to fit a particular valley on a model railroad. But before a trestle can be built, it helps to know how trestles are built. The following photos should provide excellent photo reference for trestles that would be seen on CBQ lines throughout the Midwest. The following subject was found between Hill City and Keystone on the Black Hills Central line in the Black Hills. Because this is a tourist line on a former branch of the CBQ, these trestles are built to a lighter standard than seen on mainlines.
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Figure 1.
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Figure 2.
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Supporting the railroad ties and the rail are stringer boards. These boards are at least 12 inches thick and about 6 inches wide. They are laid on end rather than flat to increase their relative strength. The number and size of these stringers would vary depending on the available lumber and the weight of the trains which would run on the line, therefore the stringers could vary quite a bit in size. On this particular trestle there are 2 stringers that are composed of 2-3 laminated boards per stringer. The boards, when paired together, provide incredible strength. The joints could also be staggered, meaning that there is no complete joint in the stringer for the entire length of the trestle.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Figure 7.
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As a re-cap, there is the terminology with definitions:
- Rail: iron railroad rail for the trains to roll freely on.
- Crosstie: lumber pieces that rail is secured too. On bridges the distance between cross ties is less than ballasted track, and trestle crossties can be small width and depth.
- Stringer: a laminated series of boards that support the crossties and run across the tops of the trestle bents. These laminated boards have staggered joints and are stood on end to increase strength.
- Runners: running parallel to the rails on top of the cross ties. Used to keep crossties properly secured.
- Bent: a vertical support in a trestle that distributes the weight of the equipment over a wide area of ground. Usually built in an "A" shape, to increase stability.
- Post: a lumber column that is part of a trestle bent. The number per bent varies, but can be anything from 3 to 12 or more, but typically 5 or 7 posts per bent.
- Cap: a piece of lumber that runs across the top of the trestle bent to distribute the weight amongst the posts in each bent. This piece of lumber also supports the stringers, which rest on top of it.
- Sway Brace: a piece of lumber bolted diagonally across the posts in a bent. These are usually paired, one on each side of the bent, which makes an "X" pattern if seen head-on. This piece stabilizes the posts, keeping them properly aligned.
- Sill: a piece of lumber bolted horizontally along the bottom or middle of the trestle bent. This provides an anchoring point for other braces and keeps the posts properly separated.
- Braces: found between the trestle bents, these pieces further stabilize the trestle bents, keeping them all aligned together. Not needed on extremely short trestles.
- Abutment: the short trestle bent found at each end of the trestle, this part of the trestle secures it to the earth, provides a transition between ballasted track and bridge track as well as acting as a retaining wall, keeping the ballast and rock from falling down underneath the trestle.
- Superstructure: The part of the trestle which directly supports the rails, includes crossties, runners, and stringers.
- Sub-structure: The part of the trestle which distributes weight to the ground, carrying the superstructure across the opening. Consists of trestle bents, abutments, braces, and pilings if necessary.
- Cribbing: A method of building retaining walls and raised pads of dirt. Square or round lumber was used with pairs being staked, but alternating their directions. The result was a Lincoln-log style retaining wall that allowed enough space for adequate drainage, but the spaces were small enough to prevent soil from washing out. Not often used today, but was popular in the 19th and early 20th Centuries.
For further reading, please see the following link. Family Garden Trains was nice enough to create an impressive article in their Primer Articles page titled Trestles 101. I highly suggest you check out their article!
> Other notes to consider is that longer bridges would have extended ties and small platforms built alongside the track. In these places, water barrels could be stored to put out fires and also provided a trapped crew member a safe place to let a train pass by them. Tool boxes could be stored on these platforms as well.
> Some bridges even had walkways alongside the tracks complete with handrails for safety.
> Where a large stream or river was encountered, a space in the trestle became necessary. Oftentimes a truss bridge, or a steel girder bridge was used to span the gap. These bridges, while not rare, are certainly a minority among trestles.
> As steel became more available, trestles were built, or rebuilt with steel. Many of these massive structures are around today all over the country, and thus can be seen even on Class I mainlines.
> Smaller wooden trestles can be found on almost every railroad in the United States, but are especially prevalent in the Midwest and in the mountain regions of the continent.
Sources of info for this article:
(BCC), CPRR.org. “Progressive Development of U.S. Railroads - 1830-1950.” Central Pacific Photographic History Museum, 24 Oct. 2004, cprr.org/Museum/RR_Development.html. Accessed 27 Aug. 2017.
(BCC), CPRR.org. “Progressive Development of U.S. Railroads - 1830-1950.” Central Pacific Photographic History Museum, 24 Oct. 2004, cprr.org/Museum/RR_Development.html. Accessed 27 Aug. 2017.
Race, Paul D. “Trestles 101.” Family Garden Trains, Amazon Company, familygardentrains.com/primer/bridges/trestle/trestle.htm. Accessed 27 Aug. 2017.
Photo copyright 2017 James Willmus.
Photo copyright 2017 James Willmus.
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