Transportation is a major contributor to global carbon emissions due to its reliance on fossil fuels, with railways often cited as a promising solution for emission reduction. However, empirical evidence of railways’ effectiveness in reducing carbon emissions has been limited. Our study reveals that the expansion of Japan’s railway network over the past 30 years has led to a significant reduction in carbon emissions, ranging from 97.44 to 110.73 million metric tons. This translates to an annual reduction of up to 1.697% of Japan’s transportation sector emissions in 2019, a finding that demonstrates the broader environmental implications of systemic railway development. In contrast, station openings have led to a slight overall increase in emissions, contributing an additional 2.5 million metric tons over the same period. These findings highlight the effectiveness of comprehensive network expansions in reducing carbon emissions, as opposed to localized station openings, and reinforce the importance of strategic railway expansion as a critical measure for mitigating carbon emissions and promoting sustainable urban development.