Capital Subdivision

The Thomas Viaduct spans the Patapsco River and Patapsco Valley between Relay, Maryland and Elkridge, Maryland, USA. It was commissioned by the Baltimore and Ohio Railroad; built between July 4, 1833, and July 4, 1835; and named for Philip E. Thomas, the company's first president. It remains the world's oldest multiple arched stone railroad bridge.At its completion, the Thomas Viaduct was the largest railroad bridge in the United States and the country's first multi-span masonry railroad bridge to be built on a curve. In 1964, it was designated as a National Historic Landmark. In 2010, the bridge was designated as a National Historic Civil Engineering Landmark by the American Society of Civil Engineers.

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.