The Thomas Viaduct spans the Patapsco River and Patapsco Valley between Relay and Elkridge, Maryland, USA. It is the first multi-span masonry railroad bridge in the United States to be built on a curve. Construction of the bridge commenced on July 4, 1833, and was completed on July 4, 1835. It was named for Philip E. Thomas, the first president of the Baltimore and Ohio Railroad (B&O), which the viaduct carried. The Thomas Viaduct was the largest bridge in the United States when it was built, and today it remains the world's largest bridge of its kind, as well as the world's oldest multiple arched stone railroad bridge. In 1964 it was designated as a National Historic Landmark. The viaduct is now owned and operated by CSX Transportation and still in use today, making it one of the oldest railroad bridges still in service. This Roman-arch stone bridge is divided into eight spans. It was designed by Benjamin Henry Latrobe, II, then B&O's assistant engineer, later the chief engineer and built by John McCartney of Ohio under the supervision of Caspar Wever, the railroad's chief of construction. McCartney received the contract after the successful completion of the Patterson Viaduct.