The Engaña Tunnel () is a never-completed railway tunnel in Spain, connecting the provinces of Burgos and Cantabria through the Cantabrian Mountains. The tunnel was part of the proposed Santander-Mediterranean railway line, an attempt by the Spanish government to connect the Bay of Biscay with the Mediterranean Sea. The construction lasted for over seventeen years, from 1941 to 1959, employing hundreds of workers, including Republican prisoners during the first years. At the time of its construction, it was the longest railway tunnel in Spain, with a length of , but was never completed as the rails were never laid.