Insights of airborne transmission within urban rail transit carriage environments after the COVID-19 pandemic

During the COVID-19 pandemic, the high-density and enclosed environments of rail transit systems have increased respiratory transmission risk, emphasizing the need to investigate droplet dispersion mechanisms and influencing factors in rail carriages. This article reviews the numerical simulation and experimental studies on respiratory droplet transmission in rail vehicles since 2020, focusing on droplet characteristics, infection risk assessment and influencing factors in dense and narrow environments. The review encompassed high-speed trains, conventional trains and subways, analysing the effects of ventilation, passenger behaviour and environmental conditions on spread of droplet. Through systematic literature search and keyword co-occurrence analysis, 27 core studies were identified. The results indicated that numerical modelling mostly employed Eulerian and Lagrangian frameworks with various turbulence models, simplifying 3D carriage geometries with seats and occupants. Key boundary conditions included mixed and new type ventilation systems and idealized human exhalation. These models simulated spherical droplets from coughing and breathing under velocities of 0.1–20 m/s. Experiments are more complex and time-consuming but can provide realistic insights. For infection risk assessment, optimizing models for special environments is essential. Advanced ventilation modes and higher ventilation rates are effective for controlling droplet transmission, while passenger behaviours could significantly affect dispersion and would require further research.