The Thomas Viaduct on the Baltimore and Ohio Railroad (B&O) was the first multi-span railroad bridge erected in the United States and the first built on a curving alignment. The viaduct remains virtually unchanged since its 1835 completion; consequently, it was declared a National Historic Landmark in 1964 and was designated a National Historic Civil Engineering Landmark by the American Society of Civil Engineers in 2011.
Benjamin Henry Latrobe II (1806–1878), an engineer with the B&O and son of the famed architect of the U.S. Capitol, laid out the Washington Branch of the B&O and designed the massive viaduct located on its route. It had to be high enough to avoid flooding of the Patapsco River while also accommodating a four-degree curve; to meet these requirements, Latrobe laid out the piers on radial lines, producing their distinctive wedge-shape, with wider sides facing the outside curve of the viaduct.
Ohioan John McCartney built the viaduct under the supervision of B&O chief of construction Caspar Wever and with Latrobe’s heavily involvement. The 612-foot-long viaduct (704 feet long including the approaches) has 8 elliptical arches with pilasters between them. The viaduct stands 59 feet tall from the water level to the base of the cast-iron decorative rail and has a 26-foot-wide deck that accommodates a double track and a 4-foot-wide pedestrian walkway. It is built of 63,000 tons of rough-dressed granite ashlar stones (obtained from nearby quarries on the Patapsco River) set in cement mortar.
The viaduct was completed in 1835 at a cost of $142,236.51 and named for Philip E. Thomas, the first president of the railroad (1827–1836). Although nicknamed “Latrobe’s Folly” because of public doubts that it would support its own weight, the viaduct defied all expectations, remaining in service to the present. In 1938, the B&O Maintenance of Way Department undertook necessary repairs, including the installation of a drainage system.
Dilts, James D. The Great Road: The Building of the Baltimore & Ohio, The Nation’s First Railroad, 1828–1853. Stanford, CA: Stanford University Press, 1993.
“Baltimore & Ohio Railroad: Thomas Viaduct.” HAER No. MD-3, Historic American Engineering Record, 1984. Prints and Photographs Division, Library of Congress, Washington, DC.