## 4 Conclusions  
The major aim of this study is to check the efficiency of the designed ventilation system for a subway station with two underground levels, retrofitted with PSDs, in the case of a train on fire stopped at the platform. The analysis is based on full-scale unsteady CFD simulations for the subway network considered. For this, a CFD model has been developed, including an approach that allows the determination of CO and CO\({}_{2}\) concentrations in the computational domain, starting from the proposed fire scenario.  
The obtained results clearly show the effect of PSDs in limiting the spread of hot and polluted air in the area of the platform, where the emergency evacuation of people should take place in the first minutes after the train on fire is stopped at the platform. Thorough investigations of the numerical results indicate that the planned emergency strategy (based on exhausting air both through the \"over track system\" and the \"under platform system\", in conjunction with opening the PSDs on the side with the train on fire) assures a tenable environment and, thus, the safe evacuation of passengers. It should be noted that this occurs throughout the entire simulation time (20 min), starting with the very first seconds. Consequently, this emergency strategy in the event of trains on fire stopped at the platform of the stations could be recommended for other stations retrofitted with PSDs in the Bucharest subway network.  
On the other hand, the results of this study should be further investigated considering other configurations (e.g., modified fire outbreak positions, multiple fire sources, different fire development, another HRR maximum power) or issues (e.g., opening the PSDs on the opposite side of the fire, starting the nearest end-of-station fan plant, problems related to the operation of ventilation systems--delayed starts, low air flow rates, etc.).  
In addition, sensitivity analysis studies for the CFD model presented in this paper should be methodically carried out in future work to assess the influence of main uncertainty sources in the model (e.g., distribution and mean value of diffusers/grills, air velocity, turbulence modeling, fire hazard) on the results. Data from these investigations will facilitate the assessment of uncertainty bands for the principal outputs of the model (e.g., pollutant concentrations). Moreover, this will allow the evaluation of the individual influence of certain model input data, considering the overall deviations of the numerical results. This is very important to fully understand the simulation results and their proper application in fire safety studies for subway stations equipped with PSDs.  
Conceptualization, C.I.T. and V.F.K.; methodology, C.I.T. and V.F.K.; software, C.I.T. and V.F.K.; validation, C.I.T. and V.F.K.; formal analysis, C.I.T. and V.F.K.; investigation, C.I.T. and V.F.K.; resources, C.I.T.; data curation, C.I.T. and V.F.K.; writing--original draft preparation, C.I.T.; writing--review and editing, C.I.T.; visualization, C.I.T.; supervision, C.I.T. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding. The APC was funded by INSTRUCT 5--CNFIS-FDI-2022-0397 project.  
Figure 29: Air velocity vectors (m/s)—longitudinal section between the subway train and the ceiling (minute 20).