The built railway infrastructure belongs to the most economically and strategically important transportation infrastructures. It includes load-bearing structures of the railway track carrying trains. The safe operation of the built railway infrastructureis of primary importance. The railway track is thus of great importance for reliable built railway infrastructure. Over the last century, the use of fixed railway track systems, mainly built in concrete, has increased as an alternative to the traditional ballasted railway track. Parallel to the development of railway infrastructure, significant development has been made in building materials research. With the introduction of the Ultra-High Performance Fiber Reinforced Cementitious Composite (UHPFRC) materials, it has become possible to fulfill one of the main criteria for crack-free structural elements in the construction of fixed railway tracks. The UHPFRC is known for its highly compact, fine-grained matrix with a maximum grain size of 0.5 mm, strengthened by a very high amount of short, slender steel fibers. UHPFRC has the capacity to transmit tensile stresses up to 7-10 MPa in the elastic domain and 10-16 MPa in the strain-hardening domain while showing high fatigue strength. UHPFRC remains crack-free under service conditions. This research first investigates the damage mechanisms of fixed railway track systems made of concrete and the interaction between bridge structure and fixed railway track. In addition, this research investigated the approach of replacing a ballasted railway track in an existing tunnel on an inhomogeneous invert by a fixed railway track. As a result, three concepts for fixed rail tracks made of UHPFRC have been developed that meet the requirements for crack-free structural elements and therefore provide a service duration of more than 100 years: -Slab-like fixed railway track systems for use in tunnels and on massive viaducts -Structural improvement of the mass-spring-damper system -Concept of the prefabricated bridge girder made in reinforced UHPFRC.

An essential focus of this research was the determination of the pull-out resistance of short anchorage in UHPFRC by means of non-linear Finite Element analysis. The findings contribute to the optimization of fixed railway track systems since the length of rail fasteners in UHPFRC can be reduced to 50 mm. These novel concepts using UHPFRC lead to original fixed railway track systems that avoid costly repair, enhance load-bearing capacity and service duration of existing structures, reduce railway track thickness (except in mass-spring-damper systems), optimize the construction process while minimizing railway line closures, enable prefabrication for short-span railway bridges, and ensure sustainable construction with lower CO2 emissions.