The railroad tunnel was constructed from 1871 to 1872 as part of the Shepaug Valley Railroad. A crew of coal miners from Pennsylvania built the 235-foot curved tunnel by hand using picks, dynamite and nitroglycerin to blast through Steep Rock Ridge. It took them only nine months to complete, a remarkably quick project in those days. The railroad ceased operation in 1948. Today, hikers can walk the trail that passes through the tunnel.