Railroad Bridges And TrestlesRailroad bridges can trace their roots back to the industry's beginnings in the mid-19th century. Early bridges were built of stone and almost all of these impressive works of engineering can be found in the eastern United States, many of which still carry trains today. However, after iron and steel began to be widely employed in the latter 19th century new bridge types began to be used the most common of which were (and still are for the most part) various designs of trusses, beams, girders and spans.
Other famed stone bridges around the country include the Pennsylvania Railroad's Rockville Bridge near Harrisburg, Pennsylvania crossing the mighty Susquehanna River. This massive structure is 3,820 feet long and includes 48 arches. it opened in 1902 and still carries Norfolk Southern freights and Amtrak passenger trains today.
Although not built of stone it should be noted that one of the most famous arched railroad bridges in the country is the Delaware, Lackawanna & Western Railroad's Tunkhannock Viaduct. Topping out at 240 feet above the valley floor and roughly a half-mile long at 2,375 feet the structure is a striking sight (made all the more impressive by Lackawanna R.R. located across the center arch). While stone bridges are renowned for their overall beauty and extremely long lifespans they are also very expensive to build. To help keep expenses down early railroads began engineering railroad bridges at first from wood and later from iron once it became available. Some of the earliest engineered railroad bridges included differing truss designs such as the Burr arch truss of 1817 and own lattice truss of 1820. One of the most common early such designs once iron was available was the Pratt truss of 1844, patented by Thomas and Caleb Pratt, and the Warren truss of 1848. These designs can still be found on some rail lines in the country even today (and some have or are in the process of being preserved).
As steel became available in the late 1800s larger and more impressive railroad bridges were constructed. Virtually all of these were of designs already on the books, simply differing variations of them such as the cantilever truss and through arch design, the latter of which is perhaps most recognized as railroad bridges go in the New York, New Haven & Hartford's Hell Gate Bridge which connects Queens and Manhattan over the East River. This impressive structure was completed in September 1916 and remains an important artery for Amtrak and freight trains today. The most common railroad bridges are probably variations of the span plate girder designs. These bridges typically only span small streams, cuts or open areas and are relatively cheap and inexpensive to build. However, even some longer railroad bridges will be partially constructed of the span or girder design, it simply depends on how the engineer has drawn up the plans for a particular structure. For an excellent general overview of common railroad bridge designs please click here.
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