Over late Summer last year, train operator Caltrain retired all 29 diesel locomotives that worked the busy route between San Francisco and San Jose, replacing them with 23 electric trains.
Professor Joshua Apte, a professor of environmental engineering and environmental health at the University of California happened to visit a Caltrain station just as the changeover was getting under way. 
He recalls: ‘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. A light bulb went off my head – I realised this would all be going away in a few weeks.’
‘A lot of these transitions happen pretty slowly. This one happened in a blink of an eye. We had the unique opportunity to capture the ancillary public health benefits.’
Moving quickly Apte, along with lead author Samuel Cliff, began sampling the air quality in and around the diesel trains before they were decommissioned. Their focus was on black carbon and to this end they installed samplers at Caltrain stations while also using portable detectors on the trains over the course of their final weeks in service.
Some of the practices around the use of the diesel engines is quite staggering. The locomotives were typically left idling at the San Francisco terminal both between trips and for several hourse overnight due safety rules requiring extra tests after shutdowns. During the study, between three and eight locomotives were often idling at the platform. Using detailed movement logs, Apte and Cliff were able estimate total daily idling time in locomotive minutes.
Their findings were published yesterday in Environmental Science and Technology Letters, showing that after electrification, passengers’ exposure to black carbon was reduced by an average of 89%. Black carbon concentrations in and around the San Francisco station also fell dramatically.
Apte said: ‘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. 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.’
Samuel Cliff said: ‘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.’
Apte added: ‘This is something that we ought to find a way to do as quickly as possible, everywhere. 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.’
The full research can be read here.