<p>With the growing urgency to address climate change, reducing greenhouse gas emissions from urban transportation systems has become a critical global challenge. Mass rapid transit systems, including rail and bus rapid transit, offer promising solutions by replacing less efficient transport modes and reducing urban air pollution. However, accurately quantifying the emission reductions achieved by MRTS projects is complex, requiring detailed consideration of baseline emissions, direct and indirect project emissions, and leakage effects such as changes in vehicle occupancy and induced traffic. This article presents a comprehensive methodology for calculating these emission reductions, grounded in established frameworks like the clean development mechanism methodology. By combining theoretical background with practical application to a Bus Rapid Transit project in México City, the article highlights key calculation steps and challenges, providing a valuable resource for researchers and policymakers aiming to promote sustainable urban mobility.</p>