Keywords: ballastless, inhomogeneous subgrade, UHPFRC, construction logistics, life-cycle cost, maintenance strategy, clearance profile, ThinCore-Track.

Existing railway tunnels often exhibit an inhomogeneous subgrade due to drill-and-blast overbreak and poorly compacted backfill. These tunnels were initially equipped with ballasted track, today, ballasted track is increasingly replaced by ballastless systems to reduce maintenance demand and improve track geometry stability. However, for conventional ballastless track in existing tunnels, an inhomogeneous subgrade typically requires additional "improvement" measures (e.g., excavation replacement, injections, levelling layers) that are difficult to implement reliably under confined tunnel logistics and that often govern both cost and long-term performance. This paper presents the concept of ThinCore-Track, a thin, rigid slab-like ballastless railway track made of ultra-high-performance fibre-reinforced cementitious composite (UHPFRC), developed specifically for tunnels with inhomogeneous subgrades. The concept is based on a structured assessment of infrastructure needs and on the analysis of the shortcomings identified in existing ballastless track systems, which were taken into account for the new design. The proposed concept was assessed using 3D finite-element modelling in ATENA, which enables the simulation of realistic material behaviour under railway operational loads and load models in accordance with standards. The results indicate crack-free behaviour under operational loads and fatigue resistance. Compared with current tunnel renewal solutions, the concept allows for the reduction of construction complexity by avoiding conventional reinforcement, multi-stage casting, and extensive subgrade treatment steps. Furthermore, its low construction height allows for an increase in the dimensions of the clearance profile, and in some cases, without having to mill the tunnel walls for this purpose. Economically, reconstruction costs are 22–36% lower than the analysed current solutions under comparable boundary conditions. Over a 100-year reference period, the life-cycle cost increase remains limited (mainly for wear components), while conventional approaches can incur intervention costs that are multiple times higher due to recurring repairs and replacements.

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