The 704-foot-long Thomas Viaduct is the oldest stone arch railroad viaduct in use today. Designed by Benjamin Henry Latrobe Jr., then a young engineer with little experience, the viaduct was named for Philip E. Thomas, first president of the B & O Railroad. At the time of its construction the viaduct was the largest bridge in America and the first built on a curve. The eight arched bridge was designed to carry newly introduced mechanical steam engines weighing as much as six tons apiece. Officially completed July 4, 1835, it now bears engines weighing about three hundred and fifty tons each, with no additional reinforcement. The Thomas Viaduct is located at Relay, the halfway point between Baltimore and Ellicotts Mills, where horses pulling rail cars were changed.

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