## 4 Discussion  
### Comparative System  
This study aims to emphasize the importance of conducting a comparative analysis of traction emissions, station operation, and infrastructure in different urban railway transportation systems across Brazil. It should be noted that there is limited availability of related studies, and the comparative analysis is conducted within the same scope whenever  
Figure 3: Composition of consumption and emissions. possible, drawing on data and information from specialized literature and various case studies with diverse local parameters [47]. The results obtained from this comparison can provide valuable insights into the potential for reducing emissions in urban railway transportation systems and contribute to the establishment of emission reduction targets in the Brazilian context. This study compares greenhouse gas emissions (GHE) from urban railway systems, including the metro systems in Recife, Sao Paulo, and Rio de Janeiro, as well as the urban railway (train) in Rio de Janeiro. Furthermore, the reasons for data discrepancies among these cities are explored. To facilitate the comparison, emission intensities for each city are standardized per passenger-kilometer traveled (PKT). Standardizing emission intensities per PKT enables fairer comparisons and allows for the identification of areas for improvement in each railway system. This approach takes into account both the number of passengers transported and the distance traveled, thus providing a more equitable evaluation of environmental performance [7]. Therefore, this comparison of emissions standardized per PKT in different urban railway systems offers valuable insights into disparities and provides important information for the development of more sustainable and emission-efficient transportation strategies. In 2021, the total GHE from electricity and diesel consumption for traction in the metro system of Pernambuco amounted to 5160.04 tCO\({}_{2}\)e, based on a total of approximately 32 million passengers, resulting in traction emissions of 4 g CO\({}_{2}\)e/PKT. In 2016, the total GHE from electricity consumption for traction in the metro system of Rio de Janeiro reached 14,963 tCO\({}_{2}\)e. With a total passenger volume of approximately 245 million, traction emissions were measured at 5.5 g CO\({}_{2}\)e/PKT. The total traction emissions in 2016 for the railway system in Rio de Janeiro were 22,273 tCO\({}_{2}\)e, with a total passenger volume of 182 million [48; 49]. Hence, traction emissions on the Rio de Janeiro railway amounted to 5.6 g CO\({}_{2}\)e/PKT [50]. According to the governance report of the metro system of Sao Paulo in 2022, traction emissions for the Sao Paulo metro were 6 g CO\({}_{2}\)e/PKT [45; 51]. In Figure 4 presents the comparison of traction emissions per PKT in these three locations. Based on these results, Pernambuco exhibited the lowest traction emissions per PKT, while the Sao Paulo metro had the highest emissions per PKT. Traction emissions are influenced by factors such as train characteristics, track conditions, and operational modes, among others. However, since these contributing factors are not the main focus of this study, they were not detailed within this research. Figure 4: Traction emission per PKT. These results can be attributed to three factors: the carbon intensity of electricity supply, passenger volume, and the efficiency of electricity use in urban railway transportation. The adopted greenhouse gas emission factors for electricity were similar across the case studies. However, there are notable differences in train flow between the Pernambuco metro and the metros in Rio de Janeiro and Sao Paulo, with much longer intervals between trains in Pernambuco. Additionally, the Pernambuco metro has a smaller network extension compared to Rio de Janeiro and Sao Paulo. Typically, higher passenger occupancy rates result in lower emissions per PKT.