| Underground subway spaces are characterized by strong enclosure and high connectivity, where fire smoke spreads rapidly and can easily cause mass casualties. Existing assessment methods struggle to simultaneously achieve dynamic quantification, temporal evolution tracking, and spatial heterogeneity identification of fire risk. In this study, a full-scale Fire Dynamics Simulator (FDS) numerical model of an A-type 6-car train and an island platform was established, and four typical fire scenarios—fires in the 2nd, 4th, and 6th carriages and a platform distribution box fire—were set up to systematically analyze the spatiotemporal evolution characteristics of smoke temperature, visibility, and CO concentration. A coupled assessment method integrating the Fire Hazard Index (FHI) and the Finite State Machine (FSM) was proposed; the FHI calculation was optimized by introducing a dominant-factor dynamic weighting mechanism and an adaptive moving average method, and the fire risk evolution patterns at different ignition points were quantified through state transition probability matrices. The results show that subway fire smoke follows a "horizontal progression–terminal stagnation" diffusion pattern, and 180–240 s after ignition is the critical time window for the deterioration of environmental parameters. Significant heterogeneity exists in risk evolution at different ignition points: carriage fire risk decreases from the head to the tail of the train, while the distribution box fire risk exhibits strong irreversibility and state solidification. Under the most unfavorable baseline condition of fixed ventilation without mechanical smoke extraction set in this study, the risk in all scenarios remained below the warning level with no cross-level jump. This method does not rely on scarce historical accident samples and can realize structured quantification and dynamic early warning of fire risk, providing a quantitative basis for the layout of subway platform smoke exhaust systems and the upgrading of intelligent fire monitoring systems. |
