## 5 Discussion
### Comparison with Existing Empirical Studies and Best Practices
The same platform design may be more or less efficient in different cities or countries, depending on factors such as train punctuality, passenger aggression/cooperation, the presence of personnel directing flows, physical distancing among passengers, or environmental conditions (e.g., an airport station, a major rail hub, or a commercial or educational setting, where travelers might carry more luggage or walk at different speeds). This reality limits the automatic transfer of conclusions to other regions;
* Partial scope in station modeling: Although the study examines platform areas and staircases in detail, a complete metro system includes mezzanines, street-level entrances, interconnecting corridors, and even internal commercial spaces that affect pedestrian flow. A partial model focusing on the platform may underestimate (or ignore) upstream or downstream bottlenecks, affecting the accuracy of predictions regarding the station's overall efficiency. Nevertheless, it is important to note several challenges that could arise in practical implementation:  
* Geotechnical and structural conditions: In highly seismic areas, or regions with complex soil conditions or systems subject to high vibration (taking into account the inherent vibration of an operational metro system), expanding a station to add lateral platforms may require significantly reinforcing the original structure. This could entail large-scale underground works, with greater-than-anticipated costs and timelines. For this reason, in practice, stations are not usually expanded directly. Rather, they tend to be duplicated at a reasonable distance;
* Space constraints in built-up environments: In dense urban areas, the additional construction of platforms or the widening of circulation corridors may be limited by adjacent buildings or existing installations (utility ducts, other transit lines, underground parking structures, etc.). This lack of space can hinder expansion. However, had the station originally been designed with modular capacity in mind, a reserve area could have been left for lateral platform growth;
* Financial planning and investment phases: While the modular concept aims to optimize long-term investment by building only what is necessary for current and medium-term projected demand, this approach directly addresses a very common issue: the inability to accurately predict actual long-term demand (50 years). Facing this reality, the modular system allows for maintaining a capacity reserve that could eventually be built out in the future when justified by demand, thus avoiding unnecessary expenses at the system's outset and only incurring them when truly required. Retaining this reserve capacity from the start of the project is highly efficient for long-term investment, ensuring a high level of service over time;
* Operational disruptions: Carrying out expansion work while the station remains in service presents a logistical challenge. Partial or total closure of key areas can affect passenger flow and cause inconvenience. For this reason, construction phases must consider temporary detours and safety measures to keep operations at acceptable levels. A phased construction model, by placing lateral platforms outside the main operational areas, enables the station to be expanded without negatively impacting its day-to-day operation. Overall, these challenges underscore that adopting a modular design requires not only the proper physical and operational layout of the station but also an integrated planning process that addresses technical, financial, and urban considerations. This ensures that the second phase of construction effectively meets actual demand as it evolves. Nonetheless,factoring in the possibility of modular construction from the project's inception makes it possible to anticipate and prepare for uncertain future scenarios without incurring additional costs, while still providing the potential for future capacity increases without disrupting service operations.