The viaduct is now owned and operated by CSX Transportation and is still in use today, making it one of the oldest railroad bridges still in service.

Construction began on July 4, 1828, with the original route following the upper branch of the Patapsco River which led west to Ellicott's Mills (later renamed Ellicott City) from the lower Patapsco which is the "Basin" (now Inner Harbor) at downtown Baltimore and the Baltimore Harbor and Port of the lower river estuary leading southeast 15 miles to flow into the Chesapeake Bay.

This new line branched at Relay, the site of a former post road hotel and changing point for stage horses.

He also celebrated the completed work by having his men kneel on the deck of the viaduct while mock "baptizing" them with a pint of whiskey.

During the period of 1834-1835, Michel Chevalier was commissioned by the French government to study the North American canal and railroad networks.

Chevalier produced a two-volume report, "Histoire et description des voies de communication aux etats-Unis (1840–1841).

[11] [12] By 1838, Franz Anton von Gerstner was the "... leading engineer and scholar of the emerging railroad industry on the continent of Europe."

Until after the American Civil War, the B&O was the only railroad into Washington, D.C.; thus, the Thomas Viaduct was essential for supply trains to reach the capital of the Union during that conflict.

To counteract the deterioration of the masonry, the Thomas Viaduct underwent more cosmetic upgrades in 1938 performed by the B&O Maintenance of Way Department.

At an unknown date, railing blocks were removed from the north side of the deck, and a bracketed walkway was added, giving more lateral clearance.

From the 1880s to the 1950s, Thomas Viaduct carried B&O's famed Royal Blue Line passenger trains between New York and Washington.