While there is a rich body of literature on the impact of high-speed rail (HSR) on aviation, research on railway speedup and the consequent environmental effects remains scarce. We apply an augmented gravity model to examine the effect of railway speedup on air travel demand and CO₂ emissions for 546 city pairs from 2013 to 2017. We observe that, on average, railway weighted operation speed increased from 100.9 km/h in 2013 to 141.2 km/h in 2017. As railway operation speed increased by 39.94%, the number of airline passengers declined by 29.84% for all market segments. During the research period, about 104.92 million air passengers shifted to rail transport due to improved rail services driven by speed, leading to a reduction of 13.57 million tons of CO₂ emissions. Our findings highlight the importance of optimizing transportation structure to reduce CO₂ emissions.