The Thomas Viaduct, named after Philip E. Thomas, first president of the Baltimore & Ohio Railroad, became an enduring symbol of the B&O Railroad and the Patapsco Valley by surviving several floods and outlasting many modern structures. In 1833, B&O engineers sought to build a first-class railroad line with gentle curves and low grades from Baltimore to Washington D.C., and spanning the Patapsco Valley posed a formidable challenge. Benjamin Latrobe, Jr. designed the bridge, completed in July 1835, and it was among the largest stone-arched bridges in the world. The 704 foot curved structure, 66 feet high, connected Relay to Elkridge on eight elliptical arches. Although critics thought it incapable of bearing its own weight, it carried the first steam locomotives and continues to support modern-day trains. Hundreds of immigrant workers built it under the hard-driven direction of John McCartney, laboring for low wages in improvised shantytowns and using hand tools, wheelbarrows, primitive pulleys, sweat, and muscle to hoist 63,000 tons of granite into place; several were killed during construction. Before 1872, the B&O's Washington Branch was the federal capital's only direct rail and telegraph link with the North, and during the Civil War protecting this connection was critical to the Union war effort. In 1861, Union troops began a four-year occupation of Relay and Elkridge to guard the railroad from Confederate raiders and saboteurs, and Union General Benjamin F. Butler used the camp at Relay as a staging area for his occupation of Baltimore's Federal Hill, securing the city for the Union.
