Real-time condition monitoring is a critical step to warrant the integrity of rail tracks in bourgeoning high-speed railway (HSR) industry. Nevertheless, existing damage identification, condition monitoring and structural health monitoring (SHM) approaches, despite their proven effectiveness in laboratory demonstration, are restricted from in-situ implementation in engineering practice. By leveraging authors’ continued endeavours, an in situ health and condition monitoring framework, using actively generated diffuse ultrasonic waves (DUWs) and a benchmark-free condition-contrasting algorithm, has been developed and deployed. Fatigue cracks in the tracks show unique contact behaviours under different conditions of external loads and further disturb DUW propagation, and the crack growth induced by external loads can also alternate DUW propagation. By contrasting DUW propagation traits, fatigue cracks in rail tracks can be characterised quantitatively and the holistic condition of the tracks can be evaluated in a real-time manner. Compared with guided wave- or acoustic emission-based methods, the DUW-driven inspection philosophy exhibits immunity to ambient noise and measurement uncertainty, less dependence on baseline signals, and high robustness in atrocious engineering conditions. Conformance tests are performed on rail tracks, in which the evolution of fatigue damage is monitored continuously and quantitatively, demonstrating effectiveness, reliability and robustness of DUW-driven condition monitoring towards HSR applications.