Risk Assessment of Positive Train Control by Using Simulation of Rare Events

The risk assessment of positive train control (PTC) presents a number of challenges that can be addressed through simulation, a common tool for analyzing large, complex stochastic systems. The combined analysis of a simulated rail system with safety models that track the propagation of human errors and equipment failures toward hazards and accidents (or their eventual safe resolution) enables the prediction of accidents and their probability of occurrence for a base case without PTC and an alternate case with PTC. Accidents are rare events, and when probabilities of rare events are estimated, efficiency is a major concern because the computer resources required for statistically reliable estimates are usually overwhelming. The problem of efficiency can be addressed through multilevel splitting, or staged simulation. The basic idea of splitting is to create separate copies of the simulation whenever it approaches the rare event. The FRA generalized train movement simulator (GTMS) integrates a rail system simulator with safety models and staged simulation to arrive at metrics of safety and risk that meet federal regulatory requirements. The simulation techniques used and a description of their implementation in the GTMS are presented. The paper concludes with a case study risk assessment that uses the GTMS of a nonvital overlay PTC system for a Class I railroad.