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To limit the impact of climate change, there is an urgent requirement for infrastructure decarbonization and a transition to lower-emission energy systems. This is expected to result in local air quality co-benefits. However, quantification of these benefits through direct observations is often difficult because most transitions involve a gradual technological substitution over years or decades. Here, we report on a unique case study of local air quality improvements resulting from the rapid and complete transition of the Caltrain commuter rail system (California, USA) from diesel to electric operation over 6 weeks in 2024. At two measurement sites within the San Francisco station, concentrations of black carbon particles (BC, a major diesel exhaust constituent) dropped by 1.9 and 0.6 μg m–3. In addition, BC exposure on board the trains decreased on average by 89% and up to 17 μg m–3 per ride, where journey direction, proximity to the locomotive, and number of stops are key factors in rail car pollutant concentrations powered by diesel engines. Given that dozens of similarly equipped, diesel-operated commuter rail systems exist across the U.S., there is substantial potential for air pollution exposure reductions elsewhere through future electrification projects.