Moderate walking between home and transit stops can be a significant portion of daily physical activity. Yet, we know little about the role of public transit in promoting physical activity at the neighborhood level. Using a pre-post treatment-control research design, we examine whether new Expo light rail transit has any impact on physical activity outcomes among residents of a low-income neighborhood in south Los Angeles. We hypothesize that physical activity gain varies by proximity to transit stations, transit trip frequency, and past behavior. The data consist of two waves of longitudinal samples collected about 6 months before and after the implementation of new Expo light rail transit in Fall 2012. Based on the 82 repeated samples across the two waves, we obtained travel behavior and physical activity data through a travel diary, GPS logger, and accelerometer for a 7-day period. The results suggest that proximity to transit has a complex association with physical activity. Taking more bus transit trips positively affects physical activity. Also, individual characteristics determine the extent to which physical activity increases. Further, past physical activity levels play a key role in moderating the effect of the Expo Line on later physical activity levels of the residents living near the Expo Line. The implication is that the health impact of transit investments at the neighborhood level is complex and potentially operates through mediators/moderators, such as personal characteristics and past behavior. Our findings indicate that health impact assessments of transit investments should be comprehensive, possibly incorporating individual and behavioral factors in order to account for this complex relationship.