The Thomas Viaduct, originally nicknamed Latrobe's Folly, spans the Patapsco River between Relay and Elkridge, Maryland. Constructed between 1833 and 1835, it is the first multi-span masonry railroad bridge in the United States and the first to be built on a curving alignment. The Thomas Viaduct was the largest bridge in the nation in its day, and was named for Philip E. Thomas, first president of the Baltimore and Ohio Railroad (B&O). The viaduct is still in use today, owned by CSX Transportation, and is the world's oldest multiple stone arched railroad bridge.

The stone, Roman-arch bridge is divided into eight spans. It was designed by Benjamin Henry Latrobe, II, then B&O assistant engineer, later chief engineer. It was built by John McCartney of Ohio under the supervision of Caspar Wever, the road's chief of construction. McCartney received the contract after the successful completion of the Patterson Viaduct. The main design problem to be overcome was that of construction on a curve: there would be variations in span and pier width between the opposite sides of the structure. This problem was solved by having the lateral pier faces laid out on radial lines, thus making the piers essentially wedge-shaped, and fitted to the 4 degree curve.

The viaduct is 612 feet (187 m) long, each arch about 58 feet (18 m); height, 59 feet (18 m) from water level to the base of the rail. The width at the top of the spandrel wall copings is 26 feet, 4 inches (8.03 m). The bridge is of rough-dressed Maryland granite ashlar, from Patapsco River quarries. A wooden-floored walkway, four feet wide and built for pedestrian and railway employee use, is supported upon cast iron brackets and is edged with ornamental cast iron railings.

In 1929, extensive pointing of the masonry was carried out, and in 1937 and 1938, major repairs were effected, including installation of a new drainage system. Nevertheless, the bridge is still indicative of the way in which, in the early days of the B&O, track and major structures were put down in the most permanent manner possible. The viaduct contains 24,476 cubic yards (18,713 m³) of masonry, and cost $142,236.51.

The original B&O route followed the Patapsco River to Ellicott's Mills; later the "Washington Branch" was constructed. This new line branched at Relay, site of a former post-road hotel and changing point for stage horses. The main line was rerouted about 1870, and since then the viaduct has carried the railroad's full traffic between Washington, D.C. and Baltimore, Maryland. Although its ability to support even its own weight was widely doubted at the time of construction, it has since carried 300-ton (270 tonnes) diesel engines and heavy freight traffic. The B&O station and hotel at Relay was demolished around 1948.

To counteract deterioration of the masory, the Thomas Viaduct underwent a "face-lifting" in 1938, performed by the B&O Maintenance of Way Department. The work consisted primarily of improved facilities for drainage, relocation of loose arch ring stones, and the application of a grout mixture to the stone spandrel filling. At an unknown date, railing blocks were removed from the north side of the deck and a bracketed walkway added, giving more lateral clearance.

When the structure was completed, a fifteen-foot (4.6 m) obelisk with the names of the builder, directors of the railroad, the architect (engineer), and others associated with the viaduct was erected at its east end by builder John McCartney. He also celebrated the completed work by having his men kneel on the deck of the viaduct, while "baptizing" them with a pint of whiskey. The official completion date for the bridge was July 4, 1835.

Until after the American Civil War, the B&O was the only railroad into Washington, and was used by Federal forces for supply trains, with heavy guards stationed along the viaduct.

Little work had been done on the viaduct until the repairs of 1937–1938 which, according to a 1949 report by the Chief Engineer of the B&O, would keep future maintenance to a minimum. The bridge was designated a National Historic Landmark on January 28, 1964, and administratively listed on the National Register of Historic Places on October 15, 1966. The Thomas Viaduct remains as impressive today as at its completion, which marked the beginning of major American railway structures.

[Source: Wikipedia]

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