The Thomas Viaduct spans the Patapsco River and Patapsco Valley between Relay and Elkridge, MD. It was commissioned by the Baltimore and Ohio Railroad (B&O); built between July 4, 1833, and July 4, 1835; and named for Philip E. Thomas, the company's first president.
At its completion, the Thomas Viaduct was the largest bridge in the United States and the country's first multi-span masonry railroad bridge to be built on a curve. It remains 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.