Abstract: Fundamental experimental data and theoretical models for fire safety assessment in specially shaped tunnels remain limited. To address this gap, a 1:12-scale physical model was developed, comprising a horseshoe-shaped tunnel and train carriages, to systematically investigate fire plume evolution and ceiling temperature distributions during train fires. The experiments reproduced the progression of a train fire from the initial single-opening breakage stage, through the development stage involving fire spillage from two openings, to the fully developed flashover stage. A range of heat release rates (HRRs) was considered to examine their effects on flame morphology, flame height, ceiling temperature, and flame-ceiling impingement behavior. A prediction model for the maximum ceiling temperature was developed and validated against previously reported experimental results. The results show that: (1) During the single-opening fire stage, flame height increases approximately linearly with HRR, while the vertical temperature distribution in front of the opening initially increases and subsequently decreases with height. (2) During the dual-opening stage, non-uniform ventilation caused by the internal configuration of the train carriage produces an asymmetric fire plume, resulting in a lateral displacement of the maximum ceiling temperature toward the direction of flame inclination. (3) During the flashover stage, once the HRR exceeds a critical threshold, continuous flame impingement on the tunnel ceiling occurs. After impinging on the ceiling, the flame spreads longitudinally along the tunnel wall in an arcuate pattern, which differs markedly from the horizontally symmetric flame spread typically observed in rectangular tunnels. (4) Under equivalent fire source conditions, horseshoe-shaped tunnels experience a greater ceiling temperature rise than rectangular tunnels.

Key words: horseshoe-shaped tunnel, train fire, window breakage, fire spillage, maximum ceiling temperature rise, flame morphology, temperature distribution