Switching from diesel to electric trains dramatically improved the air quality aboard the San Francisco Bay Area's Caltrain commuter rail line, reducing riders' exposure to the carcinogen black carbon by an average of 89%, finds a new study published today in the journal Environmental Science and Technology Letters.
The electrification of the system also significantly reduced the ambient black carbon concentrations within and around the San Francisco station, the study found.
"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," said study senior author Joshua Apte, a professor of environmental engineering and environmental health at the University of California, Berkeley. "It really adds to the case for electrifying the many other rail systems in the U.S. that still use old, poorly regulated diesel locomotives."
Caltrain operates the busiest commuter rail system in the western U.S., carrying millions of passengers a year along its 47-mile route between San Francisco and San Jose. Over the course of six weeks in August and September 2024, the system retired all 29 of its diesel locomotives and replaced them with 23 new electric trains. The debut of the new trains was the culmination of a $2.44 billion modernization and decarbonization project that first launched in 2017.
Apte, an expert in air quality monitoring , was inspired to pursue the study after visiting a Caltrain station in August 2024, when the very first electric trains were being introduced.
"I was stunned at how much the station smelled like diesel smoke and how noisy it was from the racket of diesel locomotives idling away at the platforms, dumping smoke out into the community," Apte said. "A light bulb went off my head — I realized this would all be going away in a few weeks."
After securing the support of Caltrain, Apte and study lead author Samuel Cliff quickly mobilized, installing black carbon detectors at Caltrain stations and carrying portable air quality detectors aboard the trains. For four weeks, they tracked the rapid improvements in air quality as old diesel locomotives were replaced by new electric trains.
"A lot of these transitions happen pretty slowly. This one happened in a blink of an eye," Apte said. "We had the unique opportunity to capture the ancillary public health benefits."
According to Apte and Cliff's calculations, the reduction in black carbon exposure achieved from Caltrain's electrification cut excess cancer deaths by 51 per 1 million people for riders and 330 per 1 million people for train conductors. For reference, the U.S. Environmental Protection Agency has a policy that any exposure that increases the average individual's cancer risk by more than one per million is considered 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," said Cliff, a postdoctoral scholar at UC Berkeley.
The majority of U.S. commuter trains are still powered by diesel fuel, despite the fact that electric trains are quieter, more reliable and produce fewer greenhouse gases than diesel locomotives. Apte hopes the study motivates more U.S. municipalities to follow the lead of Asian and European countries in electrifying their railways.
"This is something that we ought to find a way to do as quickly as possible, everywhere," Apte said. "California has long-term plans to electrify most of its rail systems, but this shows that we shouldn't be waiting another 25 years to get it done. We should be speeding it up."
Co-authors of the study include Haley McNamara Byrne and Allen Goldstein of UC Berkeley.
