The presentation examines signalling system engineering investigations within the rail industry focusing on how technical assurance, failure analysis and standards development underpin operational safety. It outlines how modern investigation practices are shaped by historic incidents such as Clapham, Ladbroke Grove and Potters Bar. This is where latent design, wiring, or maintenance faults combined with inadequate testing led to catastrophic outcomes. The presentation explores failure modes across assets including track circuits, points, and interlockings. Highlighting the role of human factors, configuration errors and non-compliant testing such as deviations from SMTH and SWTH. Case studies including Dalwhinnie and a recent multi-train collision in India reinforce the consequences of inadequate verification and validation. It further covers the evolution of engineering controls including structured testing handbooks, competency management and independent assurance.