Thermal Safety of Traction Batteries for Rail Transit: Chemistries, Failure Mechanisms, Mitigation Strategies, and Outlook

As rail transit accelerates its transition toward new energy systems, traction batteries, as critical energy storage and power delivery units, play a decisive role in vehicle reliability and passenger safety. This review systematically analyzes the representative operational profiles of modern rail transit systems and identifies the critical thermal safety risks faced by traction batteries under complex and dynamic loading conditions. By further distinguishing the differentiated requirements across vehicle types, we explore the interplay between electrochemical performance and safety robustness. The thermal safety behaviors of mainstream traction battery chemistries─including lithium iron phosphate (LFP) batteries, nickel-cobalt-manganese (NCM/NMC) lithium-ion batteries, lithium titanate (LTO) batteries, and supercapacitors─together with emerging systems such as sodium-ion batteries (SIBs) and solid-state lithium batteries (SSLBs), are systematically compared. The principal triggers and mechanisms of thermal runaway are summarized, and chemistry-specific failure pathways are analyzed as distinct thermal failure trajectories with their causative drivers, revealing the intrinsic weak point across different systems. Building on these insights, battery selection principles, multilevel safety management approaches, and thermal mitigation strategies tailored to rail transit operation are proposed. These insights aim to offer both theoretical guidance and technical solutions for the design and engineering of next-generation traction battery systems.