The light rail trains in Salt Lake City have operated for nearly 20 years, saving countless car trips and sparing the air tons upon tons of petroleum-powered pollutants.

Yet, over the past few years the TRAX trains have been doing more than just saving commuters time and money: they have been mapping out where and when different pollutants are present in the train’s route.

Researchers from the University of Utah found that the TRAX project is the only known transit-based mobile air quality network in North America. The trains found that spikes of carbon dioxide and nitrogen dioxide — byproducts of gasoline combustion — were found at street intersections but the trains also found methane emission patterns that didn’t correlate to daytime working hours, suggesting possible rogue methane leaks.

The researchers suggest the TRAX system could be modified to detect air pollution and greenhouse gas emission monitoring and exposure assessment on public transit systems in urban centers worldwide.

The full research can be found in the journal Atmospheric Environment.