A recent study has highlighted significant improvements in air quality following the transition from diesel to electric trains on Caltrain, a major commuter rail line in the Bay Area. The findings, which have been published in the journal Environmental Science and Technology Letters, indicate that electrification resulted in an average 89% decrease in exposure to black carbon, a known carcinogen. There was also a notable reduction in ambient black carbon levels at the San Francisco station.

According to Joshua Apte, the study's senior author and professor of environmental engineering and health at the University of California, Berkeley, "The transition from diesel to electric trains occurred over just a few weeks, and yet we saw the same drop in black carbon concentrations in the station as California cities achieved from 30 years of clean air regulations." Apte emphasizes that the results bolster the argument for electrifying other U.S. rail systems utilizing older diesel locomotives.

For the study, Apte collaborated with UC Berkeley postdoctoral scholar Samuel Cliff. During six weeks in late summer 2024, Caltrain phased out 29 diesel locomotives in favor of 23 electric trains, part of a $2.44 billion modernization initiative begun in 2017. Apte noted the stark contrast between the diesel smoke and noise observed in August 2024 and the quiet, clean environment after the transition.

The researchers utilized black carbon detectors in stations and carried portable monitors on trains to document the changes in air quality. "A lot of these transitions happen pretty slowly. This one happened in a blink of an eye," Apte explained. The reduction in black carbon levels is linked to a decreased cancer risk by 51 per million riders and 330 per million conductors, according to Apte and Cliff's calculations. For context, the U.S. Environmental Protection Agency deems exposure raising cancer risk by more than one per million as unacceptable.

"If you think about this in the context of the whole of the U.S., where we have millions of people commuting by rail every day, that’s hundreds of cases of cancer that could be prevented each year," Cliff stated. Presently, most U.S. commuter trains remain diesel-operated, despite electric trains offering benefits such as reduced noise, improved reliability, and fewer greenhouse gas emissions. Apte hopes this study encourages U.S. cities to transition towards electric rail systems similar to those in Asia and Europe. "This is something that we ought to find a way to do as quickly as possible, everywhere," he urged.

The study also saw contributions from Haley McNamara Byrne and Allen Goldstein of UC Berkeley.