Though the granite soon proved too unforgiving and expensive for track, most of the B&O's monumental bridges have survived to this day, and many are still in active railroad use by CSX. Baltimore's Carrollton Viaduct, named in honor of Charles Carroll of Carrollton, was the B&O's first bridge, and is the world's second-oldest railroad bridge still carrying trains (the world's oldest is the Skerne Bridge, Darlington, UK of 1824–1825). The Thomas Viaduct in Relay, Maryland, was the longest bridge in the United States upon its completion in 1835. It also remains in use. The B&O made extensive use of the Bollman iron truss bridge design in the mid-19th century. Its durability and ease of assembly aided faster railroad construction.