Indigo Tunnel is an abandoned railroad tunnel in Allegany County, Maryland, located about 1 mile (1.6 km) east of Little Orleans. Built by the Western Maryland Railway (WM) in 1904, Indigo was the company's longest tunnel. It was part of a major WM project to extend its rail system from Hagerstown west to Cumberland. This difficult route followed the Potomac River valley and involved construction of four additional tunnels and 23 bridges.