Tunnels are civil engineering structures with a very long design lifetime of 100 years or more. In order to assess their long-term integrity monitoring systems with high robustness, long-term stability and high accuracy are required. Furthermore, monitoring should not disturb the operation of the traffic since tunnel closures are costly. This article discusses the design, installation and first results of a distributed fibre optic monitoring system installed in the inner lining of a railway tunnel. Five individual cross sections in an anhydrite area were equipped with the fibre optic monitoring system. Anhydrite regions can cause problems over a long time as water ingress causes anhydrite material to swell, which increases loads acting on the tunnel lining. All instrumented cross sections are connected to the central communication network of the tunnel and measurements can be conducted without interfering with the tunnel traffic. The developed approach focuses on long-term monitoring, which comprises special challenges regarding hardware, data processing and independent verification measurements to evaluate the long-term stability of the measurements. Additionally, issues such as cable bites by animals or forced changes of computer operating systems must be considered to guarantee the suitability of the monitoring system over decades.