The Henryton Tunnel, located near Henryton in southern Carroll County, Maryland, is the third-oldest tunnel in the world that remains in active railroad use. Constructed by the Baltimore and Ohio (B&O) Railroad and opened around 1850, it was the first tunnel constructed on the B&O's Old Main Line. In 1865 the tunnel was widened to accommodate double track. It was rebuilt into its current form in 1903. The tunnel has brick portals and lining.