Active transportation, such as walking, biking or using public transportation, is a strategic pathway to improving physical activity levels and thus reducing excess weight. Utilizing a forthcoming expansion of the Washington D.C. area transportation system with a new 16-mile light rail line, the Purple Line Light Rail Impact on Neighborhood, Impact and Transit (PLIGHT) Study will exam pre- and post-Purple Line public transportation use, active transportation behaviors and attitudes and physical activity among Prince George’s County adults and youth.