Get Over It! Fun with Railroad Bridges
- Michael Byle
- Jul 5
- 5 min read
Every railroad has bridges. Bridges are used to cross water bodies such as rivers, bays, lakes, etc. or to cross depressions such as canyons, gulches, narrow valleys, etc. There are an infinite variety of bridge types and while many may look similar, they are often quite unique. The type of bridge is determined by the grade and curvature of the track, available materials, and technology in use by the railroad at the time of construction. Railroads were experienced in different technologies, such as reinforced concrete, cut or mortared stone, timber and not all used the same methods. Of course, where the technology could not span the water, a ferry in lieu of a bridge is also an option. The railroad’s Chief Engineer was the one who most often made the decisions.
Early bridges were often built, failed, rebuilt with improved methods. Sometimes bridges are removed and replaced with better construction to increase capacity. When rebuilding bridges, the new bridge was often built alongside of the old bridge so that rail traffic may still pass during construction. After the new bridge is complete, the tracks are realigned to meet the new bridge. Evidence of this is visible or piers being as old abutments and/or bridge piers are present along the side of an existing bridge.
Where passage of boats and ships may be needed, the bridge must have spans wide enough for the vessels to pass between piers without damaging the ship or the bridge. The span must be high enough for the ships to pass or an opening must be created. The opening can be created by a rotating span that swings out of the way of vessels, a vertical lift span between two towers, or a bascule span. A bascule is a fancy name for a leaf bridge, where a section of track pivots from one end to lift out of the way. Bascule bridges may have one or two leaves.
The condition of the foundations also determined the type of bridge used. Where sound rock is present, arched bridges can be used to span large distances. Where foundation conditions are poor, spans must be kept short to avoid overloading the foundations. Low bridges over shallow lakes often use a lot of pilings with short spans, especially where soft mud is present in the lake bed. Trestles are similar in construction, but where they are high, piers are built as pilings with cross bracing. There are many more conditions that come into play in choosing the bridge type, including economies of construction and physical challenges of the location.
Truss bridges can be economical for large spans and come in a variety of styles. The complexity of the truss is often and indicator of the size of the span and era in which it was built. Some of the basic bridge truss types are illustrated below in Figure 1. When looking at old rail bridges, it can be useful to be able to identify the type of truss used. There are other truss types, including some hybrid arched trusses. If you have the time and interest, you can become expert in identifying trusses, but mostly, modelers need only to define the one they want to build.
Another common type of bridge is the plate girder bridge. Girders are typically made from steel plate sections riveted together. They can be built with the girders below as a deck bridge, or with the track between two large girders, referred to as a through girder bridge. They can be ballasted, meaning that there is a solid deck with track laid in ballast stone, but more often are not, to reduce weight.
Many model companies produce girder bridges. The classic Lionel and American Flyer companies both sold through girder bridges as part of their accessories, but also produced truss bridges as well. Some examples are included in the photos that follow.
A number of current manufacturers make bridges as complete units or as kits. Bachmann still produces Plasticville structures, which, though they are simple snap together designs, are suprisingly realistic and can be easily modified and detailed for added realism.
Trestles are another common type of bridge structure. These are most commonly used to span valleys or other depressions in terrain. They may be connected as approaches to bridges spanning rivers. Trestle bridges are characterized by a number of short spans supported by closely spaced frames. A trestle is a frame, typically a tripod or frame constructed of pilings that are cross-braced. Each frame is called a bent. Viaducts, are similar to trestles, except they have towers supporting much longer spans at higher elevation, though some trestles have reached substantial heights.
There is much more to bridge construction and many more styles of bridges. Keep your eyes open when traveling along a rail route and I am sure you will see many more than I have described here. If you are modeling a specific section of railway, pay attention to the details of the bridges and it will greatly enhance your modeling. Even if you are not a detail modeler, it can be quite fascinating to see the ways that railroads have use spanned over obstacles to their trackwork. Enjoy!
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