4. Risk analysis methodology, railway safety principles (PSF), and the digital revolution

4.1 The railway's historical methodology is based on three stages involving PFS

Deterministically identify the dreaded mass and energy events whose consequences are unacceptable: PSF1 to PSF4 approach and fixed sizing margins.

Statistically estimate the thresholds for intervention or replacement of critical components by observing the modes and speeds of degradation, fatigue or failure, using precise statistical data (notably Weibull's law) to approximate critical failure modes. The statistical approach also forms the basis of the braking curve charts, which originated in a campaign to measure the braking of a hundred or so test trains in the 1920s by the PLM network.

The statistical approach suffers from a sometimes incomplete history...