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 (B&O); 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. 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.