This thesis is focussed on the rehabilitation of old masonry railway tunnels that were built using the "Old Austrian Tunnelling Method" with extended application to period equivalent tunnel construction methodologies, such as the English, Belgian or German methods. The thesis initially focusses on the existing ÖBB tunnels, in particular the old railway tunnels and progresses to state-of-the-art construction measures and best practice, operational boundary conditions and the cost determination for tunnel rehabilitation. The operational conditions, legislative aspects and the cost determination method used data, information, and legislation of Austria. To apply the knowledge gained, a 6250-metre-long, twin-track model tunnel built in 1908 was defined as a basis for the study. The model tunnel is subjected to a planning process similar to a preliminary project. Furthermore, the planning process illustrates the decision-making path and possible alternative options to serve as a support for similar projects. This process includes a variant study, the development of various construction designs, a construction schedule and a project cost estimation. In the planning process the need for a full renewal in sections of the model tunnel is identified with a conversion from double-track to single-track operation. Based on the results of the variant study, two variants are considered in detail, both of which are a combination of a partial closure (approx. 3-month construction period) and a full closure (approx. 6-month construction period) of the tunnel.