An intensive particulate sampling campaign was conducted in spring and summer of 2010 to compare two types of rail systems on the L.A. Metro, an underground subway system (Metro red line) and a ground-level light-rail system (Metro gold line). In general, commuters of the subway line are exposed to greater PM concentrations than commuters of the light-rail line by almost two-fold. Regression analysis showed that the light-rail line is heavily influenced by ambient PM levels and its particulate pollutants originate from local sources, such as vehicular emissions and road dust. The subway line is less influenced by ambient PM levels and has an additional source of airborne particulate pollution that is generated from the daily operation of trains. Strong correlations of PM2.5 and PM10 between train and stations reveal that PM from stations is the main source of PM inside trains.