## 2 Materials and Methods
### CFD Model
The recommended y\({}^{+}\) values for \"standard wall functions\" are 30 \(<y^{+}<\) 300, with mention that values close to 30 are considered much more appropriate [43]. The average \(y^{+}\) values for the solid borders of the computational domain in our study are as follows: walls of the metro station, y\({}^{+}\) = 33.23; subway train walls, y\({}^{+}\) = 36.52. As a result, it can be concluded that the discretization requirements were respected for a proper use of the \"standard wall functions\". * Modeling of monoxide carbon (CO) and carbon dioxide (CO\({}_{2}\)) dispersion: transport and diffusion of the CO and CO\({}_{2}\) based on solving two conservation equations of the CO and CO\({}_{2}\) mass fraction, considering a mixture of air-CO-CO\({}_{2}\) in the computational domain, with the following assumptions--the mixture and its three species are treated as ideal gas; mass transfer and heat transfer in the mixture are supposed negligible; no chemical reaction between the three gases; mixture density--ideal gas law; mixture  
Figure 11: Air velocity fields (m/s) for the three considered meshes. specific heat capacity--mixing law; mixture thermal conductivity and viscosity--kinetic theory. Moreover, CO and CO\({}_{2}\) diffusion coefficients in air have constant values as follows: \(2.0\times 10^{-5}\) m\({}^{2}\)/s (CO) and \(1.6\times 10^{-5}\) m\({}^{2}\)/s (CO\({}_{2}\)). The general form of the conservation equations is as follows (tensor notation):  
\[\rho\frac{\partial}{\partial x_{i}}(u_{i}m_{i^{\prime}})+\frac{\partial}{ \partial x_{i}}J_{i^{\prime},i}=S_{i^{\prime}} \tag{2}\]  
where the first left-hand side term represents the convection (\(\rho\)--mixture density, \(x_{i}\)--spatial coordinate, \(u_{i}\)--velocity component in \(i\) direction, \(m_{i^{\prime}}\)--species mass fraction, CO/CO\({}_{2}\)), the second left-hand side term stands for the diffusive term (\(J_{i^{\prime},i}\)--CO/CO\({}_{2}\) diffusion flux), and the right-hand side term, \(S_{i^{\prime}}\), is the source term. The term for diffusion in Equation (1) includes both molecular diffusion and turbulent diffusion, as expressed below:  
\[\frac{\partial}{\partial x_{i}}J_{i^{\prime},i}=\rho\frac{\partial}{\partial x _{i}}\left(D_{i^{\prime},m}\frac{\partial m_{i^{\prime}}}{\partial x_{i}} \right)-\frac{\partial}{\partial x_{i}}\left(-\overline{u^{\prime}_{i}m^{ \prime}_{i^{\prime}}}\right) \tag{3}\]  
where the first right-hand side term represents the molecular diffusion (\(D_{i^{\prime},m}\)--diffusion coefficient) and the second right-hand side term is the turbulent diffusion (\(u_{i^{\prime}m^{\prime}}\)--turbulent mass flux of CO/CO\({}_{2}\), \(u^{\prime}_{i}\)--velocity fluctuation). The turbulent diffusion term is determined based on Reynolds analogy through the eddy viscosity (\(\mu_{t}\)) and turbulent Schmidt number (\(Sc_{t}\)) [44]:  
\[\left(-\overline{u^{\prime}_{i}m^{\prime}_{i^{\prime}}}\right)=\frac{\mu_{t} }{Sc_{t}}\frac{\partial m_{i^{\prime}}}{\partial x_{i}} \tag{4}\]  
The turbulent Schmidt number is given by the following expression [45]:  
\[Sc_{t}=\frac{\mu_{t}}{\rho D_{t}} \tag{5}\]  
where \(D_{t}\)--turbulent mass diffusivity. The value of the turbulent Schmidt number used in the simulations, both for CO and CO\({}_{2}\), was 0.7, according to the literature data. Finally, the values of the CO/CO\({}_{2}\) source term in Equation (2) are specified according to the planned fire scenario (see Figure 5). * Radiation modeling: \"conservative discrete ordinates (DO) radiation model\". This method is based on the finite-volume discretization to transform the radiative transfer equations in the transport equations for radiation intensity [46]. This is performed for a specified number of discrete solid angles within the computational domain. The number of radiation intensity transport equations is set by the number of defined discrete solid angles for the DO radiation model. These solid angles are obtained by means of \"control angles\" that establish the angular space discretization for each octant [47].