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The use of interurban and urban trains has become the preferred choice for millions of daily commuters around the world. Despite the huge public investment for train technology and mayor rail infrastructure (e.g., tunnels), train safety is still a subject of concern. The work described herein reviews the state of the art on research related to critical velocity and backlayering conditions in tunnel fires. The review on backlayering conditions includes the effect of blockages, inclination, and the location of the fire source. The review herein focuses on experimental and theoretical research, although it excludes research studies using numerical modeling. Many studies have used scaled tunnel structures for experimental testing; nevertheless, there are various scaling challenges associated with these studies. For example, very little work has been done on flame length, fire source location, and the effect of more than one blockage, and how results on scaled experiments represent the behaviour at real-scale. The review sheds light on the current hazards associated with fires in rail tunnels.

Wind flow through urban areas is studied either by wind tunnel scale experiments or via computational fluid dynamics simulations through full-scale actual models. The large difference between the Reynolds numbers based on the geometries of actual cities and wind tunnel scale cities makes the dynamic similarity between the two models uncertain. In this study, the mean and turbulent flow parameters were investigated using a large eddy simulation for two models i.e. the actual urban area model and the wind tunnel scale (1:1000) model. Kuala Lumpur City Centre, Malaysia, was considered as the case study of an urban area. Vertical velocity profiles were plotted at five locations representing different building packing densities. The results of wind tunnel scale model largely agreed with the actual model with some discrepancies in the building vicinity and wakes. The dissimilarity of the wake patterns due to the large difference in Re was responsible for the deviations. Largest discrepancies were found in the lateral and wall-normal velocity components and turbulence stresses. The results casted a shadow on the applicability of the conclusions derived from the simulations on wind tunnel scale models to the actual urban environments they represented. The deviation between the two models should be assessed before proceeding with experimental or numerical simulations on small-sized models.