The Thomas Viaduct runs from Relay, Maryland (on the Baltimore County side of the Patapsco River) to Elkridge, Maryland (on the Howard County side of the river). The viaduct is named for the first president of the Baltimore and Ohio Railroad, Philip E. Thomas. Opened on July 4, 1835 as part of B&O's Washington Branch, it is still in use today.

Its location was established between Relay and Elkridge for several reasons. There was a fairly active seaport at Elkridge, so the town clearly needed a reliable rail connection to Baltimore and other trade centers. For its part, Relay was located at the halfway point between Baltimore and Ellicott Mills and was a natural point for railcar-pulling horses to be changed.

The viaduct was designed by an engineer who had very little experience at the time. Benjamin Henry Latrobe Jr. proposed a curved bridge to span the Patapsco, with eight arches standing 60 feet tall at the high point. The primary material used for bridge construction would be stone, which would be strong enough to prevent the viaduct from washing away should the Patapsco flood.

The Thomas Viaduct was actually quite an engineering achievement for its day. It was the first curved bridge built in America. At 704 feet (612 shore to shore), it was also the longest span constructed in the country to that point. Designed to carry new mechanical steam engines, the stone bridge was designed to bear engines that weighed as much as six tons each. Even today, it bears engines that weigh as much as 350 tons apeice-- and it has not been reinforced.

Information for this writeup came from the dim memories of my middle school history class, and from the Maryland Historical Society's website:

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