Weiler Tunnel located at the technically very difficult railroad line Waldshut – Immendingen in south-western Germany is under a permanent threat of instability effects. The portal section of the tunnel built in 1889 passes through a zone of deep slow slope deformations affecting up to 150 m high right bank slope of the Wutach River Valley. In spite of a series of attempts to stabilise the zone close to the portal of the tunnel, tunnel tube deformations reappeared. Therefore, railroad operation was closed in 1955, and after a series of improving measures, it was opened to traffic in 1977 again as a “Museum Railroad of Wutach Valley” being operated in summer tourist seasons only. Since 1950 systematic monitoring has been introduced to secure safe operation. Moderate variations of movement rates are registered at the portal section of the tunnel, dependent on long-term precipitation balance. Horizontal displacements are higher than vertical ones and reach about 12 mm/year at the maximum.