A new study published in Environmental Science and Technology Letters has revealed that switching from diesel to electric trains on the San Francisco Bay Area’s Caltrain commuter rail line has dramatically improved air quality. The research found that riders’ exposure to black carbon, a known carcinogen, decreased by an average of 89%.
The electrification of the Caltrain system also resulted in a significant reduction in ambient black carbon concentrations within and around the San Francisco station.
“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 US that still use old, poorly regulated diesel locomotives.”
Caltrain operates the busiest commuter rail system in the western US, 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 modernisation and decarbonisation 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 smelt 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 realised this would all be going away in a few weeks.”
After securing the support of Caltrain, Apte and study lead author Samuel Cliff quickly mobilised, 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.
