UIC Leaflet 779-10 is addressed to all the people involved in the maintenance of underground structures and the operation of the line. Thus, the owner, the person in charge of maintenance, the designer, and the person in charge of the railway operation will find in this document the basic information they need to guarantee the satisfactory operation in the long term of a tunnel or a number of tunnels. This leaflet gives the information that it is essential to have or know on a tunnel designed for railway trains in order to ensure the long term safety or to modify the operating characteristics. It describes the procedure to be accomplished and the steps to be followed to ensure the consistency between the different phases: assessment - diagnosis - treatment (work or other activities).